Musnad Ahmad 28:69

Yazid and 'Affan narrated to us—Yazid said: informed us, and 'Affan said: narrated to us—Hammad ibn Salamah, from Thabit al-Bunani, from Shu'ayb ibn 'Abd Allah ibn 'Amr, from his father 'Abd Allah ibn 'Amr, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said to me: "Fast one day and you shall have ten." I said: "Increase me." He said: "Fast two days and you shall have nine." I said: "Increase me." He said: "Fast three days and you shall have eight."