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  

The Messenger of Allah ﷺ said: Pray two rak'ahs before the Maghrib prayer. He then said (again): Pray two rak'ahs before the Maghrib prayer, it applies to those who wish to do so. That was because he feared that the people might treat it as sunnah. (Using translation from Abū Dāʾūd 1281)   

أَنَّ رَسُولَ اللهِ ﷺ قَالَ صَلُّوا قَبْلَ الْمَغْرِبِ رَكْعَتَيْنِ ثُمَّ قَالَ صَلُّوا قَبْلَ الْمَغْرِبِ رَكْعَتَيْنِ ثُمَّ قَالَ عِنْدَ الثَّالِثَةِ لِمَنْ شَاءَ كَرَاهِيَةَ أَنْ يَتَّخِذَهَا النَّاسُ سُنَّةً  

[Machine] The Messenger of Allah ﷺ said, "Pray two units (rak'ahs) before Maghrib prayer. Then he said, "Pray two units (rak'ahs) before Maghrib prayer for whoever fears that people may consider it a recommended act."  

قَالَ رَسُولُ اللهِ ﷺ صَلُّوا قَبْلَ الْمَغْرِبِ رَكْعَتَيْنِ ثُمَّ قَالَ صَلُّوا قَبْلَ الْمَغْرِبِ رَكْعَتَيْنِ لِمَنْ شَاءَ خَشْيَةَ أَنْ يَتَّخِذَهَا النَّاسُ سُنَّةً  

"صَلُّوا قَبْلَ المَغْرب رَكعَتَين، صلوا قَبْلَ المَغْرب رَكْعَتين لمَن شَاءَ".