Musnad Ahmad 482:30

Yahya ibn Ghaylan narrated to us, Rashdin narrated to us, Bakr ibn Amr al-Ma'afiri narrated to us, from Shu'ayb ibn Zur'ah al-Ma'afiri, that he narrated to him that he heard Uqbah ibn Amir saying: The Messenger of Allah, may Allah bless him and grant him peace, says: "Do not cause your souls to be in fear after they have been in security." They said: "And what is that, O Messenger of Allah?" He said: "Debt."