Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5712
'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.