Sahih al-Bukhari 1126
Sahih al-Bukhari · Book 19: Prayer at Night (Tahajjud) · The Prophet's (pbuh) exhorting to Tahajjud and Nawafil without making them compulsory
Narrated Um Salama:
حَدَّثَنَا ابْنُ مُقَاتِلٍ ، أَخْبَرَنَا عَبْدُ اللَّهِ ، أَخْبَرَنَا مَعْمَرٌ ، عَنِ الزُّهْرِيِّ ، عَنْ هِنْدٍ بِنْتِ الْحَارِثِ ، عَنْ أُمِّ سَلَمَةَ ـ رضى الله عنها ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم اسْتَيْقَظَ لَيْلَةً فَقَالَ " سُبْحَانَ اللَّهِ مَاذَا أُنْزِلَ اللَّيْلَةَ مِنَ الْفِتْنَةِ، مَاذَا أُنْزِلَ مِنَ الْخَزَائِنِ مَنْ يُوقِظُ صَوَاحِبَ الْحُجُرَاتِ، يَا رُبَّ كَاسِيَةٍ فِي الدُّنْيَا عَارِيَةٍ فِي الآخِرَةِ " .
One night the Prophet (ﷺ) got up and said, "Subhan Allah! How many afflictions Allah has revealed tonight and how many treasures have been sent down (disclosed). Go and wake the sleeping lady occupants of these dwellings up (for prayers), perhaps a well dressed in this world may be naked in the Hereafter."