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كتاب الصوم
30  
Fasting
(20)
Chapter: Sahur is a blessing but it is not compulsory
(20)
باب بَرَكَةِ السَّحُورِ مِنْ غَيْرِ إِيجَابٍ
Sahih al-Bukhari 1922

Narrated `Abdullah:

The Prophet (ﷺ) fasted for days continuously; the people also did the same but it was difficult for them. So, the Prophet (ﷺ) forbade them (to fast continuously for more than one day). They slid, "But you fast without break (no food was taken in the evening or in the morning)." The Prophet (ﷺ) replied, "I am not like you, for I am provided with food and drink (by Allah).

حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا جُوَيْرِيَةُ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم وَاصَلَ فَوَاصَلَ النَّاسُ فَشَقَّ عَلَيْهِمْ، فَنَهَاهُمْ‏.‏ قَالُوا إِنَّكَ تُوَاصِلُ‏.‏ قَالَ ‏ "‏ لَسْتُ كَهَيْئَتِكُمْ، إِنِّي أَظَلُّ أُطْعَمُ وَأُسْقَى ‏"‏‏.‏
Reference : Sahih al-Bukhari 1922
In-book reference : Book 30, Hadith 31
USC-MSA web (English) reference : Vol. 3, Book 31, Hadith 145
  (deprecated numbering scheme)