Sahih al-Bukhari 6671

Ishaq ibn Ibrahim narrated to me; he heard Abd al-Aziz ibn Abd al-Samad; Mansur narrated to us, from Ibrahim, from Alqamah, from Ibn Mas'ud, may Allah be pleased with him, that the Prophet of Allah, peace and blessings be upon him, led them in the noon prayer and added to it or left something out. Mansur said: I do not know whether Ibrahim or Alqamah was mistaken. It was said, ‘Messenger of Allah, was the prayer shortened, or did you forget?’ He said, ‘What is that?’ They said, ‘You prayed such-and-such.’ He then led them in two prostrations and said: ‘These two prostrations are for the one who does not know whether he added to his prayer or left something out. He should determine what is correct, complete what remains, and then perform two prostrations.’