Musnad Abu Ya'la al-Mawsili 107:6

Zuhayr narrated to us, Abdullah bin Yazid narrated to us, Haywah narrated to us, Bakr bin Amr informed me that Shu'ayb bin Zur'ah narrated to him, he said: Uqbah bin Amir narrated to me that he heard the Prophet, may God bless him and grant him peace, saying to his companions: "Do not frighten your souls," or he said, "the souls." It was said: "O Messenger of God, and how do we frighten our souls?" He said: "By debt."