Language:
كتاب التهجد
19  
Prayer at Night (Tahajjud)
(35)
Chapter: The (optional) salat before the Maghrib prayers
(35)
باب الصَّلاَةِ قَبْلَ الْمَغْرِبِ
Sahih al-Bukhari 1183

Narrated `Abdullah Al-Muzni:

The Prophet (ﷺ) said, "Pray before the Maghrib (compulsory) prayer." He (said it thrice) and in the third time, he said, "Whoever wants to offer it can do so." He said so because he did not like the people to take it as a tradition.

حَدَّثَنَا أَبُو مَعْمَرٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنِ الْحُسَيْنِ، عَنِ ابْنِ بُرَيْدَةَ، قَالَ حَدَّثَنِي عَبْدُ اللَّهِ الْمُزَنِيُّ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ صَلُّوا قَبْلَ صَلاَةِ الْمَغْرِبِ ‏"‏‏.‏ ـ قَالَ فِي الثَّالِثَةِ ـ لِمَنْ شَاءَ كَرَاهِيَةَ أَنْ يَتَّخِذَهَا النَّاسُ سُنَّةً‏.‏
Reference : Sahih al-Bukhari 1183
In-book reference : Book 19, Hadith 60
USC-MSA web (English) reference : Vol. 2, Book 21, Hadith 277
  (deprecated numbering scheme)