Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 990
Rabi'a b. Ka'b said: I was with Allah's Messenger (may peace be upon him) one night. and I brought him water and what he required. He said to me: Ask (anything you like). I said: I ask your company in Paradise. He (the Holy Prophet) said: Or anything else besides it. I said: That is all (what I require). He said: Then help me to achieve this for you by deyoting yourself often to prostration.