Chapter on the Detailed Explanation of Prayer, Including the Obligatory and Recommended, and What is Permissible and Not Permissible.

Narrated by Muhammad ibn Ahmad ibn Yahya, from Ali ibn Isma'il, from a man, from Mu‘alla ibn Khunays: I asked Abul Hasan al-Madi عليه السلام about a man who forgets a prostration in his prayer. Imam عليه السلام said: "If he remembers it before his bowing, he should perform it and continue with his prayer, then perform the two prostrations of forgetfulness (sajdatay al-sahw) after completing the prayer. But if he remembers it after his bowing, he must repeat the prayer. Forgetting a prostration in the first two rak‘ahs and the last two rak‘ahs is the same."