From Sahih Bukhari (Hadith 4965):
Narrated Abu Ubaida:I asked `Aisha 'regarding the verse:--'Verily we have granted you the Kauthar.' She replied, "The Kauthar is a river which has been given to your Prophet on the banks of which there are (tents of) hollow pearls and its utensils are as numberless as the stars
Reference: Bukhari 4965