Mustadrak al-Hakim — Hadith 1228

Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates: I was sitting with Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) while he was the Caliph, and he said: "O Ibn Abbas! Have you heard from the Messenger of Allah (peace and blessings be upon him) or from any of his Companions anything mentioning what the Messenger of Allah (peace and blessings be upon him) commanded when a man forgets in his prayer?" I said: "No, O Commander of the Believers! Have you not heard anything either?" He said: "No." Just then Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) came in to us and asked: "What are you two discussing?" Sayyiduna Umar (may Allah be pleased with him) said: "I asked him whether he has heard anything from the Messenger of Allah (peace and blessings be upon him) or his Companions about the ruling on forgetfulness in prayer." Abd al-Rahman (may Allah be pleased with him) said: "I have knowledge of that." Umar (may Allah be pleased with him) said: "(Tell us,) for you are, in our view, just and well-pleasing." So Abd al-Rahman (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When one of you is in doubt about two rak'ahs, let him count them as one; when he is in doubt between two and three, let him count them as two; and when he is in doubt between three and four, let him count them as three. Then let him complete the rest of his prayer, so that the doubt falls on the side of addition. Then let him perform two prostrations (of forgetfulness) while sitting, before giving the salam."
This hadith is sahih (authentic) on the condition of Imam Muslim, and it is a supporting witness (shahid) for the hadith of Abd al-Rahman bin Thabit bin Thawban mentioned two hadiths before this one.