Musnad Ahmad 30:3744

Abdullah ibn Yazid narrated to us, Ibn Lahi'ah narrated to us, Abdullah ibn Hubayrah narrated to me, from Abu Tamim al-Jayshani, who said: Abdullah ibn Hurmuz, a freed slave from the people of Medina, wrote to me, mentioning from Abu Hurayrah that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever follows a funeral procession and carries it from its sides, and throws soil into its grave, and sits until he is given permission [to leave], he returns with two qirats of reward, each qirat being like [the mountain of] Uhud."