From Sahih Bukhari (Hadith 2462):
Narrated `Umar:When Allah took away the soul of His Prophet at his death, the Ansar assembled In the shed of Bani Sa`ida. I said to Abu Bakr, "Let us go." So, we came to them (i.e. to Ansar) at the shed of Bani Sa`ida. (See Hadith No. 19, Vol. 5 for details)
Reference: Bukhari 2462