Hadithcore
Mishkat al-Masabih, 1165

‘Abdallah b. Mughaffal reported the Prophet (ﷺ) as saying, “Pray before the sunset prayer,” adding when saying it a third time, “This applies to those who wish to do so.” That was because he did not wish the people to treat it as a <i>sunna</i>. (Bukhari and Muslim.)

وَعَنْ عَبْدِ اللَّهِ بْنِ مُغَفَّلٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم: «صلوا قبل صَلَاة الْمغرب رَكْعَتَيْنِ صَلُّوا قَبْلَ صَلَاةِ الْمَغْرِبِ رَكْعَتَيْنِ» . قَالَ فِي الثَّالِثَةِ: «لِمَنْ شَاءَ» . كَرَاهِيَةَ أَنْ يَتَّخِذَهَا النَّاسُ سنة

References1 variant
In-Book Reference
Book 4, Hadith 582