Musnad Ahmad 28:485

He said: Abd al-Samad narrated to us, my father narrated to me, Habib—meaning al-Mu'allim—narrated to us, from Amr ibn Shu'ayb, from his father, from Abdullah ibn Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A man entered Paradise due to his leniency, both as a creditor and as a debtor."