Sahih Bukhari Volume 8, Book 73, Hadith Number 102.

Narated By ‘Aisha : (The wife of the Prophet) “I do not remember my parents believing in any religion other than the Religion (of Islam), and our being visited by Allah’s Apostle in the morning and in the evening. One day, while we were sitting in the house of Abu Bakr (my father) at noon, someone said, ‘This is Allah’s Apostle coming at an hour at which he never used to visit us.’ Abu Bakr said, ‘There must be something very urgent that has brought him at this hour.’ The Prophet said, ‘I have been allowed to go out (of Mecca) to migrate.'”