Sahih Muslim Book 031, Hadith Number 5869.

Chapter : The merits of Abu Bakr Siddiq (Allah be pleased with him).

Abu Sa'id reported that Allah's Messenger (may peace be upon him) sat on the pulpit and said: Allah gave a choice to His servant that he may opt the beauties of the world or that which is with Him and the servant chose that which was with Him. Thereupon Abu Bakr wept and he wept bitterly and said: Let our fathers and our mothers be taken as ransom for you. It was Allah's Messenger (may peace be upon him) who had been given the choice and Abu Bakr knew it better than us, and Allah's Messenger (may peace be upon him) is reported to have said: Behold, of all people the most generous toward me in regard to his companionship and his property was Abu Bakr and were I to choose anyone as my bosom friend, I would have chosen Abu Bakr as my dear friend, but (for him) I cherish Islamic brotherliness and love. There shall be left open no window in the mosque except Abu Bakr's window.