Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates: "The Messenger of Allah (peace and blessings be upon him) never fasted a complete month other than Ramadan. When he fasted, he would fast so much that someone would say: By Allah, he will not stop fasting; and when he stopped fasting, he would leave it so long that someone would say: By Allah, he will not fast."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12393