That the Messenger of Allah used to fast continually and it ws said that he would not break his fast, and he used not to fast and it was said that he would not fast.
Sunan an-Nasa'i Book 22, Hadith 270
← Previous Hadith "I said: 'O Messenger of Allah, sometimes you fast, and you hardly ever break your hardly ever fast, except two days which, if you are fasting, you include them in your fast, and if you are not fasting, then you fast them on your own.' He said: 'Which two days?' I said: 'Monday and Thursday.' He said: 'Those are two days in which deeds are shown to the Lord of the worlds, and I like my deeds to be shown (to Him) when I am fasting."'

Sunan an-Nasa'i Book 22, Hadith 269 Next Hadith → "The Messenger of Allah used to be keen to fast on Mondays and Thursday."

Sunan an-Nasa'i Book 22, Hadith 271