Narrated Mu’adh bin Jabal (RA): that the Messenger of Allah (peace be upon him) said: “Do you know what Allah’s right upon His slaves is?” I said: “Allah and His Messenger know best.” He said: “His right upon them is that they worship Him alone and do not associate any partners with Him.” He said: “And do you know what their right over Allah is if they do that?” I said: “Allah and His Messenger know best.” He said: “That He will not punish them.”
Jami`at-Tirmidhi, Book 40, Hadith 38