Musnad Abu Ya'la al-Mawsili 174:1

Dāwūd ibn ‘Amr narrated to us, Abū Shihāb al-Hannāt narrated to us, from Muhammad ibn Ishāq, from Yazīd ibn Abī Habīb, from Marthad ibn ‘Abd Allāh, from Mālik ibn Hubayrah, that when he would follow a funeral procession and meet its people, he would divide them into three rows, then pray over it. And he informed that the Messenger of Allah (peace be upon him) said: "No one prays over a dead person in three rows except that it becomes obligatory [for Paradise]."