Musnad Ahmad 696:10

'Affan narrated to us, Hammad—meaning Ibn Salamah—narrated to us, Thabit narrated to us, from 'Abd al-Rahman ibn Abi Layla, from Suhayb, that the Messenger of Allah (ﷺ) used to move his lips with something after the dawn prayer during the days of Hunayn, which we had not seen him do before. We said: "O Messenger of Allah, we see you doing something you did not use to do; what is this that you are moving your lips with?" He said: "A prophet from among those who were before you was impressed by the large number of his nation, so he said: 'Nothing will overcome these people.' Then Allah revealed to him: 'Choose for your nation one of three things: either I shall set an enemy upon them from other than themselves who will conquer them, or hunger, or I shall send death upon them.' He consulted them, and they said: 'As for the enemy, we have no strength against them; and as for hunger, we have no patience for it; but death.' So He sent death upon them, and seventy thousand of them died in three days." The Messenger of Allah (ﷺ) said: "So I am saying now, since he saw their large number: 'O Allah, by You I strive, by You I attack, and by You I fight.'"