Musnad Ahmad 340:0

It was narrated from Rabi’ah ibn Ka’b al-Aslami, who said: I was with the Messenger of Allah (ﷺ) one night, and I brought him his water for ablution and his needs. He said to me: "Ask me." I said: "I ask of you your companionship in Paradise." He said: "Or something other than that?" I said: "That is it." He said: "Then help me against your own self by performing many prostrations."