Musnad Ahmad 2:128

Abu Nuh narrated to us, Malik bin Anas narrated to us, from Zayd bin Aslam, from his father, from Umar bin al-Khattab, who said: We were with the Messenger of Allah (peace and blessings of Allah be upon him) on a journey. He said: I asked him about something three times, but he did not answer me. He said: So I said to myself, "May your mother be bereaved of you, O son of al-Khattab! You have pestered the Messenger of Allah (peace and blessings of Allah be upon him) three times, and he has not answered you." He said: So I rode my mount and went ahead, fearing that something might have been revealed regarding me. He said: Suddenly, I heard a caller calling, "O Umar! Where is Umar?" He said: So I returned, while I was thinking that something had been revealed regarding me. He said: The Prophet (peace and blessings of Allah be upon him) said: "There was revealed to me last night a Surah that is more beloved to me than the world and all that is in it: 'Indeed, We have given you a clear conquest, that Allah may forgive for you what preceded of your sin and what will follow' [Al-Fath: 1-2]."