Hadith 828:4
Waki' narrated to us, Shu'bah narrated to us, from Mu'awiyah ibn Qurrah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Fasting three days of every month is the fasting of a lifetime, and its breaking."
حدثنا وكيع، حدثنا شعبة، عن معاوية بن قرة، عن أبيه، قال: قال رسول الله صلى الله عليه وسلم: " صيام ثلاثة أيام من كل شهر، صيام الدهر وإفطاره "