Mishkat al-Masabih 1179
Al-Mukhtar b. Fulful said:
I asked Anas b. Malik about the voluntary prayers after the afternoon prayer and he replied, “Umar used to forbid prayer after the afternoon prayer, but in the time of God’s Messenger we used to pray two rak'as after the setting of the sun before the sunset prayer.” I asked him whether God’s Messenger prayed these two and he replied, “He used to see us praying them, but neither commanded us nor forbade us to do so.” 

Muslim transmitted it.
وَعَنِ الْمُخْتَارِ بْنِ فُلْفُلٍ قَالَ: سَأَلْتُ أَنَسَ بْنَ مَالِكٍ عَنِ التَّطَوُّعِ بَعْدَ الْعَصْرِ فَقَالَ: كَانَ عُمَرُ يَضْرِبُ الْأَيْدِيَ عَلَى صَلَاةٍ بَعْدَ الْعَصْرِ وَكُنَّا نُصْلِي عَلَى عَهْدِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ رَكْعَتَيْنِ بَعْدَ غُرُوبِ الشَّمْس قبل صَلَاةِ الْمَغْرِبِ فَقُلْتُ لَهُ: أَكَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّيهِمَا؟ قَالَ: كَانَ يَرَانَا نُصَلِّيهِمَا فَلَمْ يَأْمُرْنَا وَلَمْ يَنْهَنَا. رَوَاهُ مُسلم