Sahih al-Bukhari, Hadith 5081 ⓘ
Narrated 'Urwa: 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."
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، حَدَّثَنَا اللَّيْثُ، عَنْ يَزِيدَ، عَنْ عِرَاكٍ، عَنْ عُرْوَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم خَطَبَ عَائِشَةَ إِلَى أَبِي بَكْرٍ فَقَالَ لَهُ أَبُو بَكْرٍ إِنَّمَا أَنَا أَخُوكَ، فَقَالَ " أَنْتَ أَخِي فِي دِينِ اللَّهِ وَكِتَابِهِ وَهْىَ لِي حَلاَلٌ " .
also referenced under: Book 67, Hadith 19 (In-Book) & Volume 7, Book 62, Hadith 18 (USC-MSA)