During prayer, the Muslim should be God-fearing and should avoid anything that invalidates his prayer. The invalidators of prayer are:
For the praying person to utter words which do not include mentioning Allah. Uttering two letters or any letter that carries a meaning while remembering that one is praying invalidates one’s prayer.
Performing many moves that last for the duration of one rak^ah. According to some scholars, three consecutive moves invalidate the prayer.
Eating even if it is little, like a sesame seed, except if one forgets.
Drinking while remembering that one is in the prayer, even if it is little, like swallowing a drop of water on one’s lips.
Performing one move, with the purpose of playing.
Performing an excessive move such as jumping.
Purposely adding an extra integral of action, such as making sujud three times in one rak^ah.
Intending to interrupt prayer. In this case one’s prayer becomes invalid immediately.
Making the intention to interrupt prayer contingent on the occurrence of an incident, such as saying in one’s heart: “If my brother comes, I will interrupt my prayer and open the door for him.” This invalidates the prayer immediately, even if one’s brother did not come.
Hesitating about interrupting the prayer.
Completing an integral while the doubt is still persisting about one’s intention during the opening Allahu Akbar (taharrum), or doubting for a long time about this intention. If someone recites the Fatihah while doubting whether or not one intended Dhuhr or ^Asr, one’s prayer is invalid. The prayer is invalid when the time of doubt is long, even if an entire integral did not pass. An example is if one recited the entire Surah of Tabarak while one doubted one’s intention, then one’s prayer is invalid.