Musnad Ahmad 696:3

Waki’ narrated to us, from Hammad ibn Salamah, from Thabit, from ‘Abd al-Rahman ibn Abi Layla, from Suhayb, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was moving his lips during the days of Hunayn with something he had not done before that. He said: The Prophet (peace and blessings of Allah be upon him) said: "A prophet from among those who were before you was impressed by his nation, so he said: 'Nothing will overcome these people.' Then Allah revealed to him: 'Choose for them one of three things: either I unleash upon them an enemy from other than themselves who will conquer them, or hunger, or death.'" He said: "They said: 'As for killing or hunger, we have no endurance for them, but death.' He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: 'So seventy thousand died in three [days].'" He said: He then said: "So I am saying now: O Allah, by You I strive, by You I attack, and by You I fight."