Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates from Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) that the Prophet Muhammad (peace and blessings be upon him) said: "When a person makes a mistake regarding whether he has prayed two or one rak‘ah, he should consider it as one. And when he is in doubt about whether he has prayed two or three, he should consider them as two. And when he is in doubt about whether he has prayed three or four, he should consider them as three, and then complete the remaining prayer, until he becomes more inclined towards having done more (i.e., excess), not less. After that, when he is sitting (i.e., reciting Tashahhud), at that time he should perform two prostrations (i.e., Sajdah Sahw)."