[It was narrated from 'Abdullah bin Rubayyi'ah that he was with the Messenger of Allah (ﷺ) on a journey and he heard the voice of a man calling the Adhan, and he said what he said. When he reached the words:
Ashhadu anna Muhammadan Rasul-Allah (I bear witness that Muhammad is the Messenger of Allah), - Al-Hakam said, "I did not hear this from Ibn Abi Lailah" - the Messenger of Allah (ﷺ) said: "This is a shepherd of a man who is away from his family." He went down into the valley and found a shepherd, standing by a dead sheep. He said: "Do you think that this is worthless to its owners?" They said: "Yes." He said: "This world is more worthless to Allah than this (dead sheep) is to its owners."]
English reference: Vol. 1, Book 7, Hadith 666
Arabic reference: Book 7, Hadith 673