From Sahih Muslim (Hadith 2724):
Ibn Abbas (Allah be pleased with both of them) reported:The Messenger of Allah (ﷺ) did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast
Reference: Muslim 2724