Sahih Muslim Book 30, Hadith Number 5712.

Chapter : The cistern of our Apostle (may peace be upon him) and its characteristics.

‘Amir b. Sa’d b. Abu Waqqas reported: I wrote (a letter) to Jabir b. Samura (and it was sent) through my servant Nafi’ asking him to inform me about something (pertaining to the Haud Kauthar). He wrote to me: I heard him (the Holy Prophet) say: I shall be there ahead of you at the Haud Kauthar.