Musnad Ahmad 711:1

Waki’ narrated to us, he said: Shu’bah narrated to us, from al-Hakam, from ‘Abd al-Rahman ibn Abi Layla, from ‘Abd Allah ibn Rabi’ah al-Sulami, who said: The Prophet, peace and blessings of Allah be upon him, was on a journey, and he heard a caller to prayer saying: "I bear witness that there is no god but Allah." The Prophet, peace and blessings of Allah be upon him, said: "I bear witness that there is no god but Allah." He said: "I bear witness that Muhammad is the Messenger of Allah." The Prophet, peace and blessings of Allah be upon him, said: "I bear witness that I am Muhammad, the Messenger of Allah." Then the Prophet, peace and blessings of Allah be upon him, said: "You will find him to be a shepherd of sheep, or one who is away from his family." When he descended into the valley, he passed by a discarded lamb, and he said: "Do you see this one as insignificant to its owners? For the world is more insignificant to Allah than this is to its owners."