It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that once Sayyiduna Umar Farooq (may Allah be pleased with him) asked him, "O boy! Have you heard this issue from the Prophet (peace and blessings of Allah be upon him) or from any companion, that if a person has doubt in his prayer, what should he do?" While this conversation was taking place, Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) was seen coming from the front. He asked, "What are you discussing?" Sayyiduna Umar (may Allah be pleased with him) said, "I was asking this boy whether he has heard this issue from the Prophet (peace and blessings of Allah be upon him) or from any companion, that if a person has doubt in his prayer, what should he do?" Sayyiduna Abdur Rahman (may Allah be pleased with him) said, "I heard the Prophet (peace and blessings of Allah be upon him) say: 'If any of you has doubt in his prayer and does not remember whether he has prayed one rak'ah or two, then he should count it as one rak'ah. If he doubts between two and three, he should consider them as two. If he doubts between three and four, he should count them as three. After that, upon completing the prayer, before saying salam, he should perform two prostrations of forgetfulness (sujood as-sahw).'"