Musnad Ahmad 30:2063

Mu'awiyah narrated to us, he said: Abu Ishaq narrated to us, from al-A'mash, from Abu Salih, from Abu Hurayrah, who said: When a funeral was brought to the Messenger of Allah, peace and blessings of Allah be upon him, or passed by him, he would ask them: "Did he leave behind any debt?" If they said: "Yes," he would say: "Did he leave behind anything to pay it off?" If they said: "Yes," he would pray over him. But if they said: "No," he would say: "Pray over your companion."