Sahih Bukhari Volume 5, Book 57, Hadith Number 39.

Narated By Abu Huraira : Allah’s Apostle said, “Whilst a shepherd was amongst his sheep, a wolf attacked them and took away a sheep. The shepherd chased it and got that sheep freed from the wolf. The wolf turned towards the shepherd and said, ‘Who will guard the sheep on the day of wild animals when it will have no shepherd except myself?” The people said, “Glorified be Allah.” The Prophet said, “But I believe in it and so do Abu Bakr and ‘Umar although Abu Bakr and ‘Umar were not present there (at the place of the event).