The Prophet ﷺ said, "Pray before the Maghrib (compulsory) prayer." He (said it thrice) and in the third time, he said, "Whoever wants to offer it can do so." He said so because he did not like the people to take it as a tradition.  

عَنِ النَّبِيِّ ﷺ قَالَ صَلُّوا قَبْلَ صَلاَةِ الْمَغْرِبِ قَالَ فِي الثَّالِثَةِ 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)  

عَنِ النَّبِيِّ ﷺ قَالَ صَلُّوا قَبْلَ صَلاَةِ الْمَغْرِبِ قَالَ فِي الثَّالِثَةِ لِمَنْ شَاءَ كَرَاهِيَةَ أَنْ يَتَّخِذَهَا النَّاسُ سُنَّةً  

I saw the most famous people amongst the companions of the Prophet ﷺ hurrying towards the pillars at the Maghrib prayer before the Prophet ﷺ came for the prayer.  

لَقَدْ رَأَيْتُ كِبَارَ أَصْحَابِ النَّبِيِّ ﷺ يَبْتَدِرُونَ السَّوَارِيَ عِنْدَ الْمَغْرِبِ وَزَادَ شُعْبَةُ عَنْ عَمْرٍو عَنْ أَنَسٍ حَتَّى يَخْرُجَ النَّبِيُّ ﷺ  

I asked Anas b. Malik about the voluntary prayers after the afternoon prayer, and he replied: 'Umar struck hit hands on prayer observed after the 'Asr prayer and we used to observe two rak'ahs after the sun set before the evening prayer during the time of the Messenger of Allah ﷺ. I said to him: Did the Messenger of Allah ﷺ observe them? He said: He saw us observing them, but he neither commanded us nor forbade us to do so.  

كَانَ عُمَرُ يَضْرِبُ الأَيْدِي عَلَى صَلاَةٍ بَعْدَ الْعَصْرِ وَكُنَّا نُصَلِّي عَلَى عَهْدِ النَّبِيِّ ﷺ رَكْعَتَيْنِ بَعْدَ غُرُوبِ الشَّمْسِ قَبْلَ صَلاَةِ الْمَغْرِبِ فَقُلْتُ لَهُ أَكَانَ رَسُولُ اللَّهِ ﷺ صَلاَّهُمَا قَالَ كَانَ يَرَانَا نُصَلِّيهِمَا فَلَمْ يَأْمُرْنَا وَلَمْ يَنْهَنَا  

When we were in Medina, the moment the Mu'adhdhin made the call to the sunset prayer, the people hastened to the pillars of the mosque and prayed two rak'ahs with the result that any stranger coming into the mosque would think that the obligatory prayer had been observed owing to the number who were praying then.  

كُنَّا بِالْمَدِينَةِ فَإِذَا أَذَّنَ الْمُؤَذِّنُ لِصَلاَةِ الْمَغْرِبِ ابْتَدَرُوا السَّوَارِيَ فَيَرْكَعُونَ رَكْعَتَيْنِ رَكْعَتَيْنِ حَتَّى إِنَّ الرَّجُلَ الْغَرِيبَ لَيَدْخُلُ الْمَسْجِدَ فَيَحْسِبُ أَنَّ الصَّلاَةَ قَدْ صُلِّيَتْ مِنْ كَثْرَةِ مَنْ يُصَلِّيهِمَا