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

Muhammad ibn Ya’qub from Ali ibn Ibrahim from his father from Ibn Abi ‘Umayr from Hammad from al-Halabi, who said: Abu Abdillah عليه السلام was asked about a man who was forgetful and did not know whether he had performed one prostration or two. Imam عليه السلام said: "Let him perform another prostration, and after the completion of the prayer, there is no obligation upon him to perform the two prostrations of forgetfulness (sajdata al-sahw)."