Musnad Abu Ya'la al-Mawsili 155:348

Hashim narrated to us, Ubayd Allah ibn Amr narrated to us, from Zayd ibn Abi Unaysah, from Adi ibn Thabit, from Abu Hazim, from Abu Hurayrah, that the Messenger of Allah (peace be upon him) said: "Whoever walks with a funeral procession until it is finished, for him is two qirats. And whoever returns before it is finished, for him is one qirat." We said, "And what is the qirat, O Messenger of Allah?" He said, "Like Uhud."