Sahih Bukhari
Wedlock, Marriage (Nikaah)
Volume: 007 / Book Number: 062 / Hadith Number: 18
Narrated 'Ursa:
The Prophet asked Abu Bakr for 'Aisha's hand in marriage. Abu Bakr said "But I am your brother." The Prophet said, "You are my brother in Allah's religion and His Book, but she (Aisha) is lawful for me to marry."