Musannaf Ibn Abi Shaybah

Hasan says that if a person forgets to perform the prostration of recitation at the beginning of the prayer and remembers it in the second unit of prayer, then he should perform three prostrations in that unit. If he remembers after completing the prayer, he should perform one prostration before engaging in conversation, and if he remembers after conversing, then he should perform the prayer anew.

Ibrahim says that if a person forgets to perform a prostration (sajdah) in prayer, then whenever he remembers during the prayer, he should perform the prostration.

Hazrat Mujahid was asked that if a person becomes doubtful whether he has performed the prostration or not, and now he is sitting, what should he do? He replied that if you wish, perform the prostration, then when you complete the prayer, perform two prostrations of forgetfulness while sitting. And if you wish, do not perform the prostration and at the end of the prayer, perform two prostrations while sitting.