Sahih Bukhari Volume 8, Book 78, Hadith Number 628.

Narated By ‘Abdullah bin Hisham : We were with the Prophet and he was holding the hand of ‘Umar bin Al-Khattab. ‘Umar said to Him, “O Allah’s Apostle! You are dearer to me than everything except my own self.” The Prophet said, “No, by Him in Whose Hand my soul is, (you will not have complete faith) till I am dearer to you than your own self.” Then ‘Umar said to him, “However, now, by Allah, you are dearer to me than my own self.” The Prophet said, “Now, O ‘Umar, (now you are a believer).”