Musannaf Ibn Abi Shaybah 4461 — A man who forgets a prostration

Al-Hasan said: If a man forgets one of the prostrations of the prayer at the beginning of his prayer and does not remember it until he is in the last rak'ah of his prayer, he should make three prostrations in that rak'ah. If he does not remember it until he has completed his prayer, but has not yet given the salam, he should make one prostration, as long as he has not spoken; and if he has spoken, he should start the prayer afresh.