Sahih Bukhari Volume 3, Book 34, Hadith Number 348.
Narated By ‘Aisha : Rarely did the Prophet fail to visit Abu Bakr’s house everyday, either in the morning or in the evening. When the permission for migration to Medina was granted, all of a sudden the Prophet came to us at noon and Abu Bakr was informed, who said, “Certainly the Prophet has come for some urgent matter.” The Prophet said to Abu Bark, when the latter entered “Let nobody stay in your home.” Abu Bakr said, “O Allah’s Apostle! There are only my two daughters (namely ‘Aisha and Asma’) present.” The Prophet said, “I feel (am informed) that I have been granted the permission for migration.” Abu Bakr said, “I will accompany you, O Allah’s Apostle!” The Prophet said, “You will accompany me.” Abu Bakr then said “O Allah’s Apostle! I have two she-camels I have prepared specially for migration, so I offer you one of them. The Prophet said, “I have accepted it on the condition that I will pay its price.”