Sahih Bukhari, Chapter: Sales and Trade

Rarely did the Prophet (saw) fail to visit Abu Bakr's house everyday, either in the morning or in the evening. When the permission for migration to Madina was granted, all of a sudden the Prophet (saw) came to us at noon and Abu Bakr was informed, who said, "Certainly the Prophet has come for some urgent matter." The Prophet (saw) said to Abu Bakr, when the latter entered "Let nobody stay in your home." Abu Bakr said, "O Allah's Apostle! (saw) There are only my two daughters (namely Aisha and Asma) present." The Prophet (saw) 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! (saw)" The Prophet (saw) said, "You will accompany me." Abu Bakr then said "O Allah's Apostle! (saw) I have two she-camels I have prepared specially for migration, so I offer you one of them. The Prophet (saw) said, "I have accepted it on the condition that I will pay its price."