Musnad Ahmad 1153:1756

'Abd al-Samad ibn 'Abd al-Warith narrated to us, he said: Aban al-'Attar narrated to us, he said: Hisham ibn 'Urwah narrated to us, from 'Urwah, that 'Abd al-Malik ibn Marwan wrote to him asking him about things. So 'Urwah wrote to him: Peace be upon you. I praise Allah to you, the One besides whom there is no god. As for what follows: You wrote to me asking me about things—and he mentioned the hadith. He said: 'Aishah informed me: That while they were at midday in their house, and there was no one with Abu Bakr except his two daughters, 'Aishah and Asma', they were suddenly surprised by the Messenger of Allah (may Allah bless him and grant him peace) when the midday heat had set in. It was not his habit to miss a day without coming to Abu Bakr's house at the beginning of the day or at its end. When Abu Bakr saw him coming at midday, he said: "What has brought you, O Prophet of Allah? Has an event occurred?" When he entered the house to them, he said to Abu Bakr: "Have those who are with you leave." He said: "There is no one here to observe you; they are only my two daughters." He said: "Indeed, Allah the Almighty and Majestic has given me permission to depart for Medina." Abu Bakr said: "O Messenger of Allah, [is there] companionship?" He said: "Companionship." Abu Bakr said: "Take one of the two she-camels"—and they were the two she-camels that Abu Bakr had been feeding, preparing them for the departure when the Messenger of Allah (may Allah bless him and grant him peace) was given permission. So Abu Bakr gave him one of the two she-camels and said: "Take it, O Messenger of Allah, and ride it." The Messenger of Allah (may Allah bless him and grant him peace) said: "I have taken it for a price."