Musnad Ahmad 962:139

Sulayman ibn Dawud narrated to us, Shu'bah narrated to us, from Muhammad ibn 'Abd Allah ibn Abi Ya'qub, who heard Abu Nadrah, from Abu Umamah, who said: I said, "O Messenger of Allah, inform me of a deed that will cause me to enter Paradise." He said, "You must fast, for there is no equivalent to it," or he said, "There is nothing like it."