Sahih Bukhari Volume 003, Book 043, Hadith Number 642.
Narated By '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 come to them (i.e. to Ansar) at the shed of Bani Sa'ida. (See Hadith No. 19, Vol. 5 for details)