Al-Adab Al-Mufrad #945

Mu'adh said, "I was riding behind the Prophet, may Allah bless him and grant him peace, and he called out, 'Mu'adh!' 'At your service!' I replied. Then the Prophet said the same thing three times and went on, 'Do you know what is Allah's right on His slaves? That they should worship Him and not associate anything with Him.' Then he rode on for an hour.