Musnad Ahmad 928:21

Yazid ibn Harun narrated to us, Hajjaj ibn Artah informed us, from Adi ibn Thabit, from Zirr ibn Hubaysh, from Ubayy, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever follows a funeral procession until it is prayed over and it is finished, for him is two qirats. And whoever follows it until it is prayed over, for him is one qirat. And by the One in Whose hand is the soul of Muhammad, it is heavier in his scale than Uhud."