Sahih Muslim, Volume 5, Book 58, Number 244:
Narrated Abu Said Al-Khudri:
…Allah’s Messenger added, “No doubt, I am indebted to Abu Bakr more than to anybody else regarding both his companionship and his wealth. And if I had to take a Khaleel (friend) from my followers, I would certainly have taken Abu Bakr, but the fraternity of Islam is sufficient. Let no door of the Mosque remain open, except the door of Abu Bakr.”