From Sahih Muslim (Hadith 2721):
A'isha, the Mother of the Believers (Allah be pleased with her), reported that the Messenger of Allah (ﷺ) used to fast (so continuously) that we said that he would not break, and did not fast at all till we said that he would not fast. And I did not see the Messenger of Allah (ﷺ) completing the fast of a month, but that of Ramadan, and I did not see him fasting more in any other month than that of Sha'ban
Reference: Muslim 2721