Something forbidden by the Prophet (saws) is legally prohibited Sahih al-Bukhari 7368
Arabic hadith text
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.
Translation
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)