Hadithcore
Mishkat al-Masabih, 1173

Abu Huraira reported God's Messenger as saying, “If anyone prays six <i>rak'as</i> after the sunset prayer, not saying anything evil during them, they will be treated for him as equivalent to twelve years’ worship.” Tirmidhi transmitted it, saying, “This is a <i>gharib</i> tradition which I know only from the traditions of ‘Umar b. Abu Khath'am; and I heard Muhammad b. Isma'il (i.e. Bukhari) say that his traditions are rejected. He declared him to be very weak.”

وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ صَلَّى بَعْدَ الْمَغْرِبِ سِتَّ رَكَعَاتٍ لَمْ يَتَكَلَّمْ فِيمَا بَيْنَهُنَّ بِسُوءٍ عُدِلْنَ لَهُ بِعِبَادَةِ ثِنْتَيْ عَشْرَةَ سَنَةً» . رَوَاهُ التِّرْمِذِيُّ وَقَالَ: هَذَا حَدِيثٌ غَرِيبٌ لَا نَعْرِفُهُ إِلَّا مِنْ حَدِيثِ عمر بن أَبِي خَثْعَمٍ وَسَمِعْتُ مُحَمَّدَ بْنَ إِسْمَاعِيلَ يَقُولُ: هُوَ مُنكر الحَدِيث وَضَعفه جدا

References1 variant
In-Book Reference
Book 4, Hadith 590