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."
حَدَّثَنَا عَلِيُّ بْنُ حُجْرٍ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ، عَنْ حُمَيْدٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّهُ سُئِلَ عَنْ صَوْمِ النَّبِيِّ، صلى الله عليه وسلم قَالَ كَانَ يَصُومُ مِنَ الشَّهْرِ حَتَّى نَرَى أَنَّهُ لاَ يُرِيدُ أَنْ يُفْطِرَ مِنْهُ وَيُفْطِرُ حَتَّى نَرَى أَنَّهُ لاَ يُرِيدُ أَنْ يَصُومَ مِنْهُ شَيْئًا وَكُنْتَ لاَ تَشَاءُ أَنْ تَرَاهُ مِنَ اللَّيْلِ مُصَلِّيًا إِلاَّ رَأَيْتَهُ مُصَلِّيًا وَلاَ نَائِمًا إِلاَّ رَأَيْتَهُ نَائِمًا . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ .
Tirmizi
The Book on Fasting • Hadith 768