Tirmidhi Book 8, Hadith 769

حَدَّثَنَا عَلِيُّ بْنُ حُجْرٍ، حَدَّثَنَا إِسْمَاعِيل بْنُ جَعْفَرٍ، عَنْ حُمَيْدٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّهُ سُئِلَ عَنْ صَوْمِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ كَانَ يَصُومُ مِنَ الشَّهْرِ حَتَّى نَرَى أَنَّهُ لَا يُرِيدُ أَنْ يُفْطِرَ مِنْهُ، ‏‏‏‏‏‏وَيُفْطِرُ حَتَّى نَرَى أَنَّهُ لَا يُرِيدُ أَنْ يَصُومَ مِنْهُ شَيْئًا، ‏‏‏‏‏‏وَكُنْتَ لَا تَشَاءُ أَنْ تَرَاهُ مِنَ اللَّيْلِ مُصَلِّيًا إِلَّا رَأَيْتَهُ مُصَلِّيًا وَلَا نَائِمًا إِلَّا رَأَيْتَهُ نَائِمًا . قَالَ أَبُو عِيسَى:‏‏‏‏ هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ.

Jafar bin Humaid narrated that: 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.