Mishkat al-Masabih 30 Fine Qualities كتاب المناقب 176

وَعَنْ أَبِي هُرَيْرَةَ قَالَ: كَانَ جَعْفَرٌ يُحِبُّ الْمَسَاكِينَ وَيَجْلِسُ إِلَيْهِمْ وَيُحَدِّثُهُمْ وَيُحَدِّثُونَهُ وَكَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُكَنِّيهِ بِأَبِي الْمَسَاكِين. رَوَاهُ التِّرْمِذِيّ

Abu Huraira told that Ja’far used to love the poor. He would sit with them and talk to them and they would talk to him. God’s messenger used to address him by the kunya Abul Masakin (father of the poor). Tirmidhi transmitted it.