The Prophet's Fasts at times other than Ramadan; and it is recommended to ensure that no month is free of fasting Sahih Muslim 1157 a
Translation
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.