From Sahih Bukhari (Hadith 5081):
Narrated 'Urwa:The Prophet (ﷺ) asked Abu Bakr for `Aisha's hand in marriage. Abu Bakr said "But I am your brother." The Prophet (ﷺ) said, "You are my brother in Allah's religion and His Book, but she (Aisha) is lawful for me to marry
Reference: Bukhari 5081