Sahih Bukhari Volume 5, Book 59, Hadith Number 588.

Narrated By Ibn Abbas  : ‘Umar used to admit me (into his house) along with the old men who had fought in the Badr battle. Some of them said (to ‘Umar), “Why do you allow this young man to enter with us, while we have sons of his own age? ” ‘Umar said, “You know what person he is.” One day ‘Umar called them and called me along with them, I had thought he called me on that day to show them something about me (i.e. my knowledge). ‘Umar asked them, “What do you say about (the Sura): “When comes the help of Allah and the Conquest (of Mecca) And you see mankind entering the Religion of Allah (i.e. Islam) in crowds. ‘So celebrate the Praises Of your Lord and ask for His forgiveness, Truly, He is the One Who accepts repentance and forgives.” (110.1-3)

Some of them replied, “We are ordered to praise Allah and repent to Him if we are helped and granted victory.” Some said, “We do not know.” Others kept quiet. ‘Umar then said to me, “Do you say similarly?” I said, “No.” ‘Umar said “What do you say then?” I said, “This Verse indicates the approaching of the death of Allah’s Apostle of which Allah informed him. When comes the help of Allah and the Conquest, i.e. the Conquest of Mecca that will be the sign of your Prophet’s) approaching death, so testify the uniqueness of your Lord (i.e. Allah) and praise Him and repent to Him as He is ready to forgive.” On that, ‘Umar said, “I do not know about it anything other than what you know.”