Anas bin Malik was asked about the Prophet's fasting and he said: "He would fast during a month until we thought that he did not want to abstain from fasting during any of it. And, he would not fast during a month until we thought that he did not want to fast during any of it. (There was no time) that I wanted to see if he was performing Salat during the night, except that I would see him praying, nor to see him sleeping, except that I would see him sleeping." (Using translation from Tirmidhī 769)
عَنْ صَلَاةِ النَّبِيِّ ﷺ وَصَوْمِهِ تَطَوُّعًا قَالَ كَانَ يَصُومُ مِنَ الشَّهْرِ حَتَّى نَقُولَ مَا يُرِيدُ أَنْ يُفْطِرَ مِنْهُ شَيْئًا وَيُفْطِرُ حَتَّى نَقُولَ مَا يُرِيدُ أَنْ يَصُومَ مِنْهُ شَيْئًا وَمَا كُنَّا نَشَاءُ أَنْ نَرَاهُ مِنَ اللَّيْلِ مُصَلِّيًا إِلَّا رَأَيْنَاهُ وَلَا نَرَاهُ نَائِمًا إِلَّا رَأَيْنَاهُ