Sahih al-Bukhari 7368
Narrated `Abdullah Al Muzam:
The Prophet (ﷺ) said, "Perform (an optional) prayer before Maghrib prayer." (He repeated it thrice) and 
the third time he said, "Whoever wants to offer it can do so," lest the people should take it as a Sunna 
(tradition). (See Hadith No. 277, Vol. 2)
حَدَّثَنَا أَبُو مَعْمَرٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنِ الْحُسَيْنِ، عَنِ ابْنِ بُرَيْدَةَ، 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.‏