Anas b. Malik said:
The Messenger of Allah(may peace be upon him) dozed for a short while and raised his smiling. He either said to them(people) or they said to him: Messenger of Allah! Why did you laugh? He said: A surah has been revealed to me just now, and then he recited: “In the name of Allah, Most Gracious. Most Merciful. To thee We have granted the fount (of abundance)” up to the end. When he recited, he asked: Do you know what al-kauthar is? They replied: Allah and his Apostle know best. He said: It is a river which my Lord, the Exalted, has promised me( to grant) in Paradise: there is abundance of good and upon it there is a pond which my people will approach on the Day of Resurrection. There are vessels as numerous as stars(in the sky).

Other references

English translation	Book 41, Hadith 4729
In-book reference	Book 42, Hadith 152
Reference	Sunan Abi Dawud 4747