Prayer is the first pillar of Islam that the Prophet (peace be upon him) mentioned after mentioning the testimony of faith, by which one becomes a Muslim. It was made obligatory upon all the prophets and for all peoples. Allah has declared its obligatory status under majestic circumstances.
The things that invalidate the prayer are well known, but the number differs according to differences among the fuqaha’. They are as follows:
1) That which invalidates wudoo’, such as breaking wind or eating camel meat.
2) Uncovering the ‘awrah deliberately. But if it is uncovered by accident and what is uncovered is only a little, or if a lot becomes uncovered but he covers it immediately, then the prayer is not invalidated.
3) Turning away from the qiblah to a large extent.
4) Presence of najaasah (impurity) on one’s body or clothes, or in the place where one is praying. If he notices it or remembers it during the prayer and removes it immediately, then his prayer is valid. Similarly, if he does not find out about it until after the prayer is over, his prayer is still valid.
5) Excessive continuous movement during the prayer for no essential reason.
6) Omitting one of the pillars (essential parts) of the prayer, such as bowing and prostration.
7) Deliberately doing an extra pillar, such as bowing.
8) Deliberately doing some pillars before others.
9) Deliberately saying the salaam before completing the prayer.
10) Deliberately changing the meaning whilst reciting.
11) Deliberately omitting one of the obligatory parts of prayer, such as the first tashahhud. But if a person forgets, his prayer is valid but he must do the prostration of forgetfulness (sujood al-sahw).
12) Deciding to stop praying.
13) Laughing out loud. Merely smiling does not invalidate the prayer.
14) Deliberately speaking. If one speaks by mistake or out of ignorance of the ruling, the prayer is not invalidated.
15) Eating and drinking.
See: Daleel al-Taalib li Nayl al-Mataalib by Shaykh Mar’i ibn Yoosuf al-Hanbali (p. 34). And Duroos Muhimmah by Shaykh Ibn Baaz.
And Allaah knows best.