بُلُوغُ الْمَرَامِ
Bulugh al-Maram 361
The Book of Prayer
وَعَنْ عَبْدِ اَللَّهِ بْنِ مُغَفَّلٍ الْمُزَنِيِّ - رضى الله عنه - عَنْ اَلنَّبِيِّ - صلى الله عليه وسلم -قَالَ : { صَلُّوا قَبْلَ اَلْمَغْرِبِ , صَلُّوا قَبْلَ اَلْمَغْرِبِ " ثُمَّ قَالَ فِي اَلثَّالِثَةِ : " لِمَنْ شَاءَ " كَرَاهِيَةَ أَنْ يَتَّخِذَهَا اَلنَّاسُ سُنَّةً } رَوَاهُ اَلْبُخَارِيُّ . [1]
Narrated 'Abdullah bin Mughaffal al-Muzani (RA): The Prophet (ﷺ) said: "Pray before Maghrib, pray before Maghrib" then he said at the third time, "Whoever wishes (to)". He said so, because he did not like the people to take it as a Sunnah. [Reported by al-Bukhari].
Sunnah.com reference: Book 2, Hadith 264