Sayyiduna Hakim bin Hizam (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him) about fasting, and he (peace and blessings be upon him) said: "Fast three days every month." I said: I am capable of more than that, until I pressed him; so he (peace and blessings be upon him) said: "Fast three days of the month." I said again: I am capable of more than that, until I pressed him; so he (peace and blessings be upon him) said: "Fast the fast of Dawud (peace be upon him): fast one day and break your fast one day."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3110