Musnad Ahmad 696:7

He said: Affan narrated to us from his book, he said: Sulayman—meaning Ibn al-Mughirah—narrated to us, he said: Thabit narrated to us, from Abd al-Rahman ibn Abi Layla, from Suhayb, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) prayed, he would whisper something that we did not understand, and he would not tell us about it. He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Did you notice me?" A speaker said: Yes. He said: "I remembered a prophet from among the prophets who was given soldiers from his people. He said: 'Who can match these, or who can stand up to these?'—or a word similar to this, Sulayman was uncertain. He said: 'Allah revealed to him: Choose for your people one of three things: either I unleash upon them an enemy from other than themselves, or hunger, or death.' He said: 'He consulted his people regarding that, and they said: You are the Prophet of Allah, we entrust that to you, so choose for us.' He said: 'He stood for his prayer.' He said: 'And they used to turn to prayer when they were distressed.' He said: 'He prayed, then said: As for an enemy from other than themselves, no; or hunger, no; but death.' He said: 'So death was unleashed upon them for three days, and seventy thousand of them died. So my whispering that you see is me saying: O Allah, O Lord, by You I fight, and by You I attack, and there is no power and no strength except by Allah.'"