Musnad Ahmad 430:12

Abu al-Nadr narrated to us, he said: al-Mubarak narrated to us, from al-Hasan, from Abdullah ibn Mughaffal, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever follows a funeral procession until he prays over it, then for him is a qirat. And whoever waits for it until it is finished, then for him are two qirats."