Sahih Muslim Book 4, Hadith Number 989.

Chapter : What is to be recited in bowing and prostration.

Ma’dan b. Talha reported: I met Thauban, the freed slave of Allah’s Messenger (may peace be upon him), and asked him to tell me about an act for which, if I do it, Allah will admit me to Paradise, or I asked about the act which was loved most by Allah. He gave no reply. I again asked and he gave no reply. I asked him for the third time, and he said: I asked Allah’s Messenger (may peace be upon him) about that and he said: Make frequent prostrations before Allah, for you will not make one prostration without raising you a degree because of it, and removing a sin from you, because of it. Ma’dan said that then lie met Abu al-Darda’ and when he asked him, he received a reply similar to that given by Thauban.