It is narrated from Hazrat Anas bin Malik (may Allah be pleased with him) regarding the interpretation of Surah Al-Kawthar that the Messenger of Allah (peace and blessings be upon him) said: It is a river in Paradise. He said: I looked at it and saw that on its banks were tents made of pearls. I asked Jibreel (peace be upon him), "What is this?" He replied, "This is the river Kawthar which Allah has granted to you."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12675