Sahih Bukhari Volume 8, Book 76, Hadith Number 590.

Narated By ‘Uqba bin ‘Amir : Once the Prophet went out and offered the funeral prayers for the martyrs of Uhud, and then went to the pulpit and said, “I am a predecessor for you and I am a witness for you: and by Allah, I am looking at my Fount just now, and the keys of the treasures of the earth (or the keys of the earth) have been given to me: and by Allah, I am not afraid that you will worship others besides Allah after me, but I am afraid that you will strive and struggle against each other over these treasures of the world.”