Sahih al-Bukhari 1141
Sahih al-Bukhari · Book 19: Prayer at Night (Tahajjud) · The waking up of the Prophet ﷺ from his sleep for the night prayer and what (how much) was cancelled from the night prayer
Narrated Anas bin Malik:
حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ عَبْدِ اللَّهِ ، قَالَ حَدَّثَنِي مُحَمَّدُ بْنُ جَعْفَرٍ ، عَنْ حُمَيْدٍ ، أَنَّهُ سَمِعَ أَنَسًا ـ رضى الله عنه ـ يَقُولُ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُفْطِرُ مِنَ الشَّهْرِ حَتَّى نَظُنَّ أَنْ لاَ يَصُومَ مِنْهُ، وَيَصُومُ حَتَّى نَظُنَّ أَنْ لاَ يُفْطِرَ مِنْهُ شَيْئًا، وَكَانَ لاَ تَشَاءُ أَنْ تَرَاهُ مِنَ اللَّيْلِ مُصَلِّيًا إِلاَّ رَأَيْتَهُ وَلاَ نَائِمًا إِلاَّ رَأَيْتَهُ. تَابَعَهُ سُلَيْمَانُ وَأَبُو خَالِدٍ الأَحْمَرُ عَنْ حُمَيْدٍ .
Sometimes Allah's Messenger (ﷺ) would not fast (for so many days) that we thought that he would not fast that month and he sometimes used to fast (for so many days) that we thought he would not leave fasting throughout that month and (as regards his prayer and sleep at night), if you wanted to see him praying at night, you could see him praying and if you wanted to see him sleeping, you could see him sleeping.