Sahih al-Bukhari 7045 (Book 96, Hadith 53)

Narrated Ibn ‘Abbas: I used to teach Qur’an to ‘Abdur-Rahman bin Auf. When Umar performed his last Hajj, ‘Abdur-Rahman said (to me) at Mina, “Would that you had seen Chief of the believers today! A man came to him and said, “So-and-so has said, “If Chief of the Believers died, we will give the oath of allegiance to such-and-such person,’ ‘Umar said, ‘I will get up tonight and warn those who want to usurp the people’s rights.’ I said, ‘Do not do so, for the season (of Hajj) gathers the riffraff mob who will form the majority of your audience, and I am afraid that they will not understand (the meaning of) your saying properly and may spread (an incorrect statement) everywhere. You should wait till we reach Medina, the place of migration and the place of the Sunna (the Prophet’s Traditions). There you will meet the companions of Allah’s Messenger (ﷺ) from the Muhajirin and the Ansar who will understand your statement and place it in its proper position’ ‘Umar said, ‘By Allah, I shall do so the first time I stand (to address the people) in Medina.’ When we reached Medina, ‘Umar (in a Friday Khutba-sermon) said, “No doubt, Allah sent Muhammad with the Truth and revealed to him the Book (Quran), and among what was revealed, was the Verse of Ar-Rajm (stoning adulterers to death).'” (See Hadith No. 817,Vol. 8)