Musnad Ahmad 962:13

'Abd al-Samad narrated to us, Shu'bah narrated to us, Muhammad ibn Abi Ya'qub al-Dabbi narrated to us, he said: I heard Abu Nasr narrating from Raja' ibn Haywah, from Abu Umamah, who said: I came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "Command me with an action that will cause me to enter Paradise." He said: "You must fast, for there is nothing equal to it." Then I came to him a second time, and he said to me: "You must fast."