Sunan an-Nasa'i 4684

It was narrated that Muhammad bin Jahsh said:"We were sitting with the Messenger of Allah when he raised his head toward the sky, and put his palm on his forehead, then he said: 'Subhan Allah, what a stern warning has been revealed! We fell silent and were scared. The following day I asked him: 'O Messenger of Allah, what is this stern warning that has been revealed? He said: 'By the One in Whose hand is my soul, if a man were to be killed in the cause of Allah then brought back to life, then killed, but he owed a debt, he would not enter paradise until his debt was paid off