Sahih al-Bukhari 7368
Sahih al-Bukhari · Book 96: Holding Fast to the Qur'an and Sunnah · Something forbidden by the Prophet (saws) is legally prohibited
Narrated `Abdullah Al Muzam:
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.
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)