Mu'adh bin Jabal, may Allah be pleased with him, narrated:
I was riding behind the prophet (may peace be upon him) and there was nothing between him and me but the part of the saddle, when he said: Mu'adh bin Jabal, to which I replied: At your beck and call, and at your pleasure, Messenger of Allah. He moved along for a while, when again he said: Mu'adh bin Jabal, to which I replied: At your beck and call, and at your pleasure, Messenger of Allah. He then again moved along for a while and said: Mu'adh bin Jabal, to which I replied: At your beck and call, and at your pleasure, Messenger of Allah. He (the Holy Prophet) asked: Do you know what right has Allah upon His servants? I said: Allah and His Messenger know best. He (the Holy Prophet) said: Verily the right of Allah over His servants is that they should worship Him, not associating anything with Him. He (the Holy Prophet), with Mu'adh behind him, moved along for a while and said: Mu'adh bin Jabal, to which I replied: At your beck and call, and at your pleasure, Messenger of Allah. He (the Holy Prophet) asked: Do you know the servants' rights upon Allah in case they do it (i.e. they worship Allah without associating anything with Him)? I (Mu'adh bin Jabal) replied: Allah and His Messenger know best. (Upon this) he (the Holy Prophet) remarked: That He would not torment them (with the fire of Hell).
Hadith number 43 in Sahih Muslim