Sahih Muslim 836
Sahih Muslim · Book 6: The Book of Prayer - Travellers · It is recommended to pray two rak`ah before Maghrib
Mukhtar b. Fulful said:
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، وَأَبُو كُرَيْبٍ جَمِيعًا عَنِ ابْنِ فُضَيْلٍ ، - قَالَ أَبُو بَكْرٍ حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ ، - عَنْ مُخْتَارِ بْنِ فُلْفُلٍ ، قَالَ سَأَلْتُ أَنَسَ بْنَ مَالِكٍ عَنِ التَّطَوُّعِ، بَعْدَ الْعَصْرِ فَقَالَ كَانَ عُمَرُ يَضْرِبُ الأَيْدِي عَلَى صَلاَةٍ بَعْدَ الْعَصْرِ وَكُنَّا نُصَلِّي عَلَى عَهْدِ النَّبِيِّ صلى الله عليه وسلم رَكْعَتَيْنِ بَعْدَ غُرُوبِ الشَّمْسِ قَبْلَ صَلاَةِ الْمَغْرِبِ . فَقُلْتُ لَهُ أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم صَلاَّهُمَا قَالَ كَانَ يَرَانَا نُصَلِّيهِمَا . فَلَمْ يَأْمُرْنَا وَلَمْ يَنْهَنَا.
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.