What teaching did the Prophet Muhammad (saw) give Rabi'a b. Ka'b to ensure his entry into Paradise?
 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.