Riyad as-Salihin 1170
Anas (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) used to leave off observing Saum (fasting) during a month until we thought that he would not observe Saum at all during it; and (sometimes) he would observe Saum till we began to think that he would not omit any day of that month. If one wished to see him performing Salat during the night, he could do that; and if one wished to see him sleeping at night, he could do that. [Al- Bukhari] .
وعن أنس رضي الله عنه، قال‏:‏ كان رسول الله صلى الله عليه وسلم ، يفطر من الشهر حتى نظن أن  لا يصوم منه، ويصوم حتى نظن أن لا يفطر منه شيئًا، وكان لا تشاء أن تراه من الليل مصليا إلا رأيته، ولا نائمًا إلا رأيته‏.‏ ‏(‏‏(‏رواه البخاري‏)‏‏)‏‏.‏