Musnad Abu Ya'la al-Mawsili 152:332

And from Muhammad ibn Bishr and al-Abbas ibn al-Fadl, they said: Sa'id ibn Abi 'Aruba narrated to us, he said: Qatadah narrated to us, from Abu al-Ahwas, from Abdullah, who said: While we were with the Messenger of Allah (may Allah bless him and grant him peace) on one of his journeys, we heard a caller calling: "Allah is the Greatest, Allah is the Greatest." The Prophet (may Allah bless him and grant him peace) said: "Upon the natural disposition (fitrah)." Then he said: "I bear witness that there is no god but Allah, I bear witness that there is no god but Allah." The Prophet (may Allah bless him and grant him peace) said: "He has exited from the Fire." So we rushed toward him, and behold, he was a man with livestock who had been overtaken by the prayer, so he called out for it.