Musnad Ahmad 30:1829

Sulayman ibn Dawud al-Hashimi narrated to us, he said: Abu Zubayd narrated to us, from al-A'mash, from Abu Salih, from Abu Hurayrah, who said: When a funeral procession passed by the Messenger of Allah (peace and blessings of Allah be upon him), he would ask them: "Does he have any debt?" If they said: "Yes," he would say: "Did he leave behind anything for its repayment?" If they said: "Yes," he would pray over him. Otherwise, he would say: "Pray over your companion."