Sahih Bukhari Volume 8, Book 76, Hadith Number 434.

Narated By ‘Uqba bin ‘Amir : The Prophet went out and offered the funeral prayer for the martyrs of the (battle of) Uhud and then ascended the pulpit and said, “I am your predecessor and I am a witness against you. By Allah, I am now looking at my Tank-lake (Al-Kauthar) and I have been given the keys of the treasures of the earth (or the keys of the earth). By Allah! I am not afraid that after me you will worship others besides Allah, but I am afraid that you will start competing for (the pleasures of) this world.”