Mishkat al-Masabih 19 Jihad كتاب الجهاد 125

وَعَنْ جَابِرٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَتَخَلَّفُ فِي الْمَسِيرِ فَيُزْجِي الضَّعِيفَ وَيُرْدِفُ ويدْعو لَهُم. رَوَاهُ أَبُو دَاوُد

Jabir told that God’s Messenger used to keep to the rear when traveling and urge on the weak. He would take someone up behind him and would make supplication for them all. Abu Dawud transmitted it.