While I was a companion rider with the Prophet (ﷺ) he said, "O Mu`adh!" I replied, "Labbaik wa Sa`daik." He repeated this call three times and then said, "Do you know what Allah's Right on His slaves is?" I replied, "No." He said, Allah's Right on His slaves is that they should worship Him (Alone) and should not join partners in worship with Him." He said, "O Mu`adh!" I replied, "Labbaik wa Sa`daik." He said, "Do you know what the right of (Allah's) salves on Allah is, if they do that (worship Him Alone and join none in His worship)? It is that He will not punish them."

(another chain through Mu'adh)

Sahih al-Bukhari Book 79, Hadith 41