Sahih Muslim 30 a

It is narrated on the authority of Mu'adh b. Jabal: I was riding behind the Prophet (ﷺ) and there was nothing between him and me but the rear part of the saddle, when he said: Mu'adh b. Jabal: To which I replied: At your beck and call, and at your pleasure, Messenger of Allah! He moved along for a few minutes, when again he said: Mu'adh b. Jabal: To which I replied: At your beck and call, and at your pleasure, Messenger of Allah! He then again moved along for a few minutes and said: Mu'adh b. Jabal: To which I replied. At your beck and call, and at your pleasure. Messenger of Allah He, (the Prophet) said: Do you know what right has Allah upon His servants? I said: Allah and His Messenger know best. He (the Prophet) said: Verily the right of Allah over His servants is that they should worship Him, not associating anything with Him. He (the Prophet) with Mu'adh behind him, moved along for a few minutes and said: Mu'adh b. Jabal: To which I replied: At your beck and call, and at your pleasure, Messenger of Allah! He (the Prophet) said: Do you know what rights have servants upon Allah in case they do it (i. e. they worship Allah without associating anything with Him)? I (Mu'adh b. Jabal) replied: Allah and His Messenger know best. (Upon this) he (the Prophet) remarked: That He would not torment them (with the fire of Hell).