Abu Dawud Book 2, Hadith Number 1315.

Chapter : The time when the prophet peacebeuponhim used to get up at night for prayer.

Narated By Rabi’ah b, Ka’b al-Aslami : I used to live with the Apostle of Allah (PBUH) at night; I would bring water for his ablution and his need. He said: Ask me. I said: Your company is in Paradise. He said: Is there anything other than that? I said: It is only that. He said: Help me for yourself by making prostration abundantly.