Musnad Ahmad 26:311

Muhammad ibn Bishr narrated to us, Sa’id narrated to us, Qatadah and ‘Abd al-Wahhab narrated to us, from Ibn Abi ‘Arubah, from Qatadah, from Abu al-Ahwas, from ‘Abdullah ibn Mas’ud, who said: While we were with the Messenger of Allah (peace and blessings of Allah be upon him) on one of his journeys, we heard a caller calling: "Allah is the Greatest, Allah is the Greatest." The Prophet of Allah (peace and blessings of Allah be upon him) said: "Upon the natural disposition (fitrah)." Then he said: "I bear witness that there is no god but Allah." The Prophet of Allah (peace and blessings of Allah be upon him) said: "He has exited the Fire." He said: So we rushed toward him, and behold, he was a man with livestock who had been overtaken by the prayer, so he called out with it.