`Ali reported God's messenger as saying, "A man called al-Harith, a farmer, will come forth from Ma Wara' an-Nahr whose army will be led by a man called Mansur who will establish (or, consolidate) things for Muhammad's family as Quraish consolidated them for God's messenger. Every believer must help him." Or he said, "respond to his summons." Abu Dawud transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ عَلِيٍّ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " يَخْرُجُ رَجُلٌ مِنْ وَرَاءِ النَّهْرِ يُقَالُ لَهُ: الْحَارِثُ حَرَّاثٌ عَلَى مُقَدِّمَتِهِ رَجُلٌ يُقَالُ لَهُ: مَنْصُورٌ يُوَطِّنُ أَوْ يُمَكِّنُ لِآلِ مُحَمَّدٍ كَمَا مَكَّنَتْ قُرَيْشٌ لِرَسُولِ اللَّهِ وَجَبَ عَلَى كُلِّ مُؤْمِنٍ نَصْرُهُ - أَوْ قَالَ: إِجَابَتُهُ - ". رَوَاهُ أَبُو دَاوُدَ
sunnah.com