Musannaf Ibn Abi Shayba 36:508

Ghundar narrated to us, from Shu'bah, from Sa'd ibn Ibrahim, who said: I heard Ubayd Allah ibn Abd Allah ibn Utbah narrating from Ibn Abbas, from Abd al-Rahman ibn Awf, who said: Umar performed Hajj and intended to deliver a sermon to the people. Abd al-Rahman ibn Awf said: "The rabble and the lowest of the people have gathered around you, so delay that until you reach Medina." He said: When I arrived in Medina, I drew near to the pulpit and heard him saying: "I have come to know that some people are saying that the caliphate of Abu Bakr was a *falta* (a sudden, unplanned occurrence). Indeed, it was a *falta*, but Allah protected [us] from its evil. There is no caliphate except by consultation."