Every believer must aid the Mahdī
The Prophet ﷺ said: A man called al-Harith ibn Harrath will come forth from Ma Wara an-Nahr. His army will be led by a man called Mansur who will establish or consolidate things for Muhammad's family as Quraysh consolidated them for the Messenger of Allah ﷺ. Every believer must help him, or he said: respond to his sermons.
النَّبِيُّ ﷺ يَخْرُجُ رَجُلٌ مِنْ وَرَاءِ النَّهْرِ يُقَالُ لَهُ الْحَارِثُ بْنُ حَرَّاثٍ عَلَى مُقَدِّمَتِهِ رَجُلٌ يُقَالُ لَهُ مَنْصُورٌ يُوَطِّئُ أَوْ يُمَكِّنُ لآلِ مُحَمَّدٍ كَمَا مَكَّنَتْ قُرَيْشٌ لِرَسُولِ اللَّهِ ﷺ وَجَبَ عَلَى كُلِّ مُؤْمِنٍ نَصْرُهُ أَوْ قَالَ إِجَابَتُهُ