Sahih Bukhari Volume 7, Book 62, Hadith Number 75.

Narrated By Abdullah bin Umar : “When Hafsa became a widow,” Umar said, “I met Abu Bakr and said to him, ‘If you wish I will marry Hafsa bint ‘Umar to you.’ I waited for a few days then Allah’s Apostle asked for her hand. Later Abu Bakr met me and said, ‘Nothing stopped me from returning to you concerning your offer except that I knew that Allah’s Apostle had mentioned (his wish to marry) her, and I could never let out the secret of Allah’s Apostle. If he had left her, I would have accepted her.'”