Language:
كتاب التهجد
19  
Prayer at Night (Tahajjud)
(5)
Chapter: The Prophet's (pbuh) exhorting to Tahajjud and Nawafil without making them compulsory
(5)
باب تَحْرِيضِ النَّبِيِّ صلى الله عليه وسلم عَلَى صَلاَةِ اللَّيْلِ وَالنَّوَافِلِ مِنْ غَيْرِ إِيجَابٍ
Sahih al-Bukhari 1129

Narrated `Aisha the mother of the faithful believers:

One night Allah's Messenger (ﷺ) offered the prayer in the Mosque and the people followed him. The next night he also offered the prayer and too many people gathered. On the third and the fourth nights more people gathered, but Allah's Messenger (ﷺ) did not come out to them. In the morning he said, "I saw what you were doing and nothing but the fear that it (i.e. the prayer) might be enjoined on you, stopped me from coming to you." And that happened in the month of Ramadan.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ ـ رضى الله عنها أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم صَلَّى ذَاتَ لَيْلَةٍ فِي الْمَسْجِدِ فَصَلَّى بِصَلاَتِهِ نَاسٌ، ثُمَّ صَلَّى مِنَ الْقَابِلَةِ فَكَثُرَ النَّاسُ، ثُمَّ اجْتَمَعُوا مِنَ اللَّيْلَةِ الثَّالِثَةِ أَوِ الرَّابِعَةِ، فَلَمْ يَخْرُجْ إِلَيْهِمْ رَسُولُ اللَّهِ صلى الله عليه وسلم، فَلَمَّا أَصْبَحَ قَالَ ‏ "‏ قَدْ رَأَيْتُ الَّذِي صَنَعْتُمْ وَلَمْ يَمْنَعْنِي مِنَ الْخُرُوجِ إِلَيْكُمْ إِلاَّ أَنِّي خَشِيتُ أَنْ تُفْرَضَ عَلَيْكُمْ ‏"‏، وَذَلِكَ فِي رَمَضَانَ‏.‏
Reference : Sahih al-Bukhari 1129
In-book reference : Book 19, Hadith 9
USC-MSA web (English) reference : Vol. 2, Book 21, Hadith 229
  (deprecated numbering scheme)