Musnad Ahmad 790:162

Aswad ibn Amir narrated to us, Abu Bakr narrated to us, from al-A’mash, from Abu Dawud, from Imran ibn Husayn, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever has a right [a debt] owed to him by a man, and he delays it [the repayment], he shall have for every day the reward of a charity."