Musannaf Ibn Abi Shayba 30:144

Ali ibn Mushir narrated to us, from al-Mukhtar, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Al-Kawthar is a river that my Lord promised me in Paradise. Upon it is much goodness. It is a basin to which my nation will come on the Day of Resurrection; its vessels are [equal to] the number of the stars."