Musannaf Ibn Abi Shayba 3:2037

4414. Abdullah bin Numayr narrated to us, from Muhammad bin Ishaq, from Makhul, that the Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you doubts in his prayer and does not know whether he increased or decreased, if he doubts between one and two, let him consider it one, so that the uncertainty is regarding an increase. Then he should prostrate twice while he is sitting before he performs the taslim, then he should perform the taslim." Muhammad said: Husayn bin Abdullah said to me, "Did Makhul attribute the hadith to you?" Muhammad said: "I asked him about that, and he said: 'Indeed, he mentioned it from Kurayb, from Ibn Abbas, that Amr bin Ayyash and another disputed about it. Then Abd al-Rahman bin Awf came and said: I heard this hadith from the Messenger of Allah.'"