Abu Dawud Book 35, Hadith Number 4730.

Narated By Anas ibn Malik : When the Prophet of Allah (PBUH) was lifted to the heavens (for travelling) in Paradise, or as he said, a river whose banks were of transparent or hollowed pearls was presented to him. The angel who was with him struck it with his hand and took out musk. Muhammad (PBUH) then asked the angel who was with him: What is this? He replied: It is al-Kawthar which Allah has given you.