Introduction to Fiqh-us-Sunnah
Ibn al-Munzhir says: "The people of knowledge agree that if one intentionally eats or drinks during a fard salah, he is to repeat the salah. The same is the case with nawafil according to the majority of scholars as what invalidates an obligatory (fard) prayer also invalidates a voluntary (nafl) prayer."
Volume 2, Page 95b: Speaking intentionally about something unrelated to the salah invalidates the salahIntentionally speaking during the salah, if it is not beneficial to the salah, invalidates the salah.
Zaid ibn Arqam relates: "We used to talk while we were in salah and a person would speak to the person next to him until the verse was revealed: 'And stand before Allah in devout obedience' and we were then commanded to observe silence during the salah." This is related by the group.
Ibn Mas'ud reports: "We used to greet the Messenger of Allah while he was in salah and he would respond to our greeting. When we returned from Abyssinia, we greeted him [during prayer] but he did not respond to our salutation. We said to him: 'O Messenger of Allah, we used to greet you while you were in salah and you used to respond to us!' He then said: 'Prayer demands one's complete attention.'" This is related by al-Bukhari and Muslim.
If one is ignorant of this ruling or speaks due to the fact that he has forgotten this ruling, his salah will still be valid
Mu'awiyyah ibn alHakam said: "I was praying behind the Messenger of Allah and someone in the congregation sneezed. I said [to him]: 'May Allah have mercy upon you.' The people then stared at me, showing their disapproval of my act. I said: 'Woe to me, why do you stare at me so?' They started to strike their hands on their thighs and when I saw that they wanted me to become silent, I was angered but said nothing. When the Messenger of Allah finished the prayer - and may my father and mother be ransomed for him, I found no teacher better than him either before or after him - he did not scold, beat, or revile me but he simply said: 'Talking to others is not seemly during the salah, for the salah is for glorifying Allah, extolling His Greatness, and reciting the Qur'an.'" This is related by Ahmad, Muslim, Abu Dawud, and an-Nasa'i. Mu'awiyyah ibn al-Hakam spoke out of ignorance of this ruling and the Prophet sallallahu alehi wasallam did not order him to repeat his salah.
Talking [if it is a reminder as to the incompleteness of the salah], does not nullify the salah as can be seen in the following hadith. Abu Hurairah says: "The Messenger of Allah led us in either the noon or after-noon prayers and he made the taslim after praying just two rak'at. Zhul Yadain said to the Prophet: 'O Messenger of Allah, has the salah been shortened or have you forgotten [part of it]?' The Prophet sallallahu alehi wassalam said: 'It has not been shortened, nor did I forget any part of it." He said: 'Yes, O Messenger of Allah, you did forget.' Thereupon the Prophet asked (the people): 'Is Zhul Yadain correct in what he says?' The people said: 'He is correct, you offered only two rak'at.' Then, the Prophet prayed the two remaining rak'at and made the taslim, said the takbir and performed the sajdah, sat and made the takbir and performed the sajdah again, and finally said the takbir and sat again." This is related by al-Bukhari and Muslim.
The Maliki school allows talking during the prayer if it is done for any good of the salah as long as it does not become a common practice and (is done) only when saying subhanallah fails to alert the imam to correct his mistake. Al-Auza'i's comments are: "Whoever intentionally speaks during the salah, seeking some benefit to the salah, does not invalidate his salah." He said that if a person recites aloud in the 'asr and someone behind him says: "It is the 'asr," (i.e., the recital is not to be aloud) then the latter person would not invalidate his salah.
Volume 2, Page 96: Intentionally making many motionsThe scholars differ over what exactly constitutes a few motions and what constitutes many motions. Some say that one makes many motions when, if seen from behind, one would be certain that he was not performing salah, and anything less than that amount is considered only a few motions. Some say that it is any act or string of actions which would make others believe that the person is not praying.
An-Nawawi says: "If a person performs a lot of actions that are not part of the salah, he invalidates his salah, and, on this point, there is no difference of opinion. If the acts are few, then they do not invalidate the salah and, on this point, there also is no difference of opinion. This is the exact position. However, there does exist a difference of opinion over what exactly constitutes a few actions and many actions, [and there exist four opinions on this point..." He says that the fourth opinion is the correct and most popular opinion. The fourth opinion is that the exact definitions of too much and too little are determined by generally accepted standards. One is not harmed in his salah by common acts such as nodding in reply to a salutation, taking off one's shoes, raising the headdress and putting it back in place, putting on or taking off a light garment, carrying or holding a small child, preventing someone from passing in front of the person in prayer, covering one's spittle in one's clothing and similar other actions. As for the other acts, those which are considered to constitute many actions (e.g., taking many consecutive steps, performing actions repeatedly) they invalidate the prayer. An-Nawawi also says: "The scholars are in agreement that many actions invalidate the prayer if they are performed consecutively [i.e., one after another]. If one separates the actions, for instance, taking a step and then stopping for a while, then taking another step or two, and then another two steps, after a pause (though a short one) between them, then the salah will not be harmed, even if he (in this manner should take a hundred or more steps. There is no difference of opinion on this point. As for light actions," he continues, "such as, moving one's finger in glorifying Allah or in itching, and so forth., these do not invalidate the prayer according to the well-known, authentic opinion, even when they are done repeatedly and consecutively, but they are disliked." AshShaf'i, in a statement concerning it, says: "Even if one counts the verses on one's fingers, it would not invalidate one's salah, but it is best to avoid [such an act]."
Volume 2, Page 97: Intentionally leaving out an essential act or condition of the prayer without any valid excuse for doing soAl-Bukhari and Muslim record that the Prophet sallallahu alehi wasallam told a bedouin who had not performed his salah well: "Return and pray for you have not prayed." (This hadith was mentioned earlier.)
Ibn Rushd writes: "There is an agreement that if one prays and he is not in a state of purity, it is obligatory for him to repeat the prayer, [that is true if the act was done] intentionally or out of forgetfulness. Similarly, one who prays without facing the qiblah, intentionally or due to forgetfulness, [must repeat the salah]. In general, if any of the conditions for the correctness of the salah are absent, it becomes obligatory to repeat the salah. "
Volume 2, Page 98: Smiling or laughing during the salahIbn al-Munzhir records that there is a consensus of opinion that laughing (during the salah) invalidates the prayer. An-Nawawi says: "This is the case if one laughs aloud, and produces sound. Most of the scholars say that there is no problem with smiling. If one is overcome by laughter and cannot control it, his salah will not become invalid if it is of minor nature. If it is a hearty laughter, it will invalidate the salah. Custom would determine whether it is a major or a minor laughter."