Musannaf Ibn Abi Shayba 21:388

Abu Usama narrated to us, Sulayman ibn al-Mughira narrated to us, Thabit narrated to us, from Abd al-Rahman ibn Abi Layla, from Suhayb, who said: When the Messenger of Allah (peace be upon him) prayed, he would whisper something that he would not inform us of. We said, "O Messenger of Allah, you whisper something when you pray that we do not understand." He said, "Have you noticed me?" I said, "Yes." He said, "I remembered a prophet from among the prophets who was given soldiers from his people. He said, 'Who can equal these?' It was said to him, 'Choose one of three things for your people: either that an enemy from other than them be empowered over them, or hunger, or death.' He presented that to his people, and they said, 'You are the Prophet of Allah, so choose for us.' He stood up to pray, and they were such that when they were alarmed, they would take refuge in prayer. He prayed and said, 'O Allah, if You empower an enemy over them from other than them, then no; or hunger, then no; but death.' So death was empowered over them, and seventy thousand of them died in three days." He said, "My whispering that you hear is that I say: 'O Allah, by You I strive, and by You I attack, and there is no power and no strength except by You.'"