Musnad Ahmad 27:351

Waki’ narrated to us, from Sufyan, from Abu al-Yaqzan, from Zadhan, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Three [types of people] will be upon mounds of musk on the Day of Resurrection: A man who led a people in prayer while they were pleased with him, a man who called the Adhan for five prayers every day and night, and a slave who fulfilled the right of Allah the Exalted and the right of his masters."