What invalidates the prayer and the prostration of forgetfulness

A man greeted the Prophet (may Allah's peace and blessings be upon him) improperly during prayer; he taught the salam upon oneself with the hand, then said:

Prayer is only fitting with the remembrance of Allah.Muslim 539

Prayer is a dialogue: whatever breaks it renders it invalid.

What invalidates the prayer

• Speaking deliberately, even two words (al-Bukhari 752); returning the salam does not invalidate.
• Eating or drinking deliberately, however little.
• Abundant movements no longer belonging to the prayer.
• Leaving a pillar deliberately (the Fatiha, the bowing, the prostration).
• Laughing aloud: it also breaks wudu for the majority.
• Turning the whole face away from the qibla.

What is excused

Unintentional errors, forgetfulness, and small gestures of the eye or hand (Muslim 286). The child learning, the trembling sick: their effort counts.

The prostration of forgetfulness (sujud as-sahw)

If one of you doubts in his prayer, let him cast off the doubt and take what he judges most likely, then complete it, then prostrate twice.Muslim 571

It compensates three cases:

• Doing more (adding a rak'a or a tashahhud): sujud after the salam.
• Doing less (forgetting the first tashahhud): sujud before the salam.
• Doubt: take the most likely view and complete, then sujud after the salam.

After the sujud: tashahhud, prayer upon the Prophet, salam. The two prostrations repair what came before (Muslim 571).