Musnad Ahmad 756:1

Waki’ narrated to us, Shu’bah’s son al-Aswad narrated to us, from Abu Nawfal ibn Abi ‘Aqrab, from his father, who said: I asked the Prophet, may Allah bless him and grant him peace, about fasting. He said: "Fast one day of the month." I said: "O Messenger of Allah, I am stronger [than that]." The Messenger of Allah, may Allah bless him and grant him peace, said: "I am stronger, I am stronger. Fast two days of every month." I said: "O Messenger of Allah, increase me." The Messenger of Allah, may Allah bless him and grant him peace, said: "Increase me, increase me. Three days of every month."