Musnad Ahmad 1153:2069

Rawh narrated to us, Kahmas narrated to us, from Abdullah ibn Shaqiq, who said: I asked Aisha, "Did the Messenger of Allah, may Allah bless him and grant him peace, fast an entire month?" She said: "I do not know of him fasting an entire month until he would break his fast during it, except for Ramadan. And he did not leave fasting an entire month until he would fast during it, until he passed away or departed on his path, may Allah bless him and grant him peace."