Sahih Bukhari Volume 007, Book 072, Hadith Number 698.
Narrated By 'Aisha : Some Muslim men emigrated to Ethiopia whereupon Abu Bakr also prepared himself for the emigration, but the Prophet said (to him), "Wait, for I hope that Allah will allow me also to emigrate." Abu Bakr said, "Let my father and mother be sacrificed for you. Do you hope that (emigration)?" The Prophet said, 'Yes." So Abu Bakr waited to accompany the Prophet and fed two she-camels he had on the leaves of As-Samur tree regularly for four months One day while we were sitting in our house at midday, someone said to Abu Bakr, "Here is Allah's Apostle, coming with his head and a part of his face covered with a cloth-covering at an hour he never used to come to us." Abu Bakr said, "Let my father and mother be sacrificed for you, (O Prophet)! An urgent matter must have brought you here at this hour." The Prophet came and asked the permission to enter, and he was allowed. The Prophet entered and said to Abu Bakr, "Let those who are with you, go out." Abu Bakr replied, "(There is no stranger); they are your family. Let my father be sacrificed for you, O Allah's Apostle!" The Prophet said, "I have been allowed to leave (Mecca)." Abu Bakr said, " I shall accompany you, O Allah's Apostles, Let my father be sacrificed for you!" The Prophet said, "Yes," Abu Bakr said, 'O Allah's Apostles! Let my father be sacrificed for you. Take one of these two she-camels of mine" The Prophet said. I will take it only after paying its price." So we prepared their baggage and put their journey food In a leather bag. And Asma' bint Abu Bakr cut a piece of her girdle and tied the mouth of the leather bag with it. That is why she was called Dhat-an-Nitaqaln. Then the Prophet and Abu Bakr went to a cave in a mountain called Thour and remained there for three nights. 'Abdullah bin Abu Bakr. who was a young intelligent man. used to stay with them at night and leave before dawn so that in the morning, he would he with the Quraish at Mecca as if he had spent the night among them. If he heard of any plot contrived by the Quraish against the Prophet and Abu Bakr, he would understand it and (return to) inform them of it when it became dark. 'Amir bin Fuhaira, the freed slave of Abu Bakr used to graze a flock of milch sheep for them and he used to take those sheep to them when an hour had passed after the 'Isha prayer. They would sleep soundly till 'Amir bin Fuhaira awakened them when it was still dark. He used to do that in each of those three nights.