Sahih Bukhari Volume 007, Book 072, Hadith Number 850.
Narrated By Mu'adh bin Jabal : While I was riding behind the Prophet and between me and him and between me and him there was only the back of the saddle, he said, "0 Mu'adh!" I replied, "Labbaik, 0 Allah's Apostle, and Sa'daik!" he said, "Do you know what is Allah's right upon his slave?" I said, "Allah and His Apostle know best" He said "Allah's right upon his slaves is that they should worship Him alone and not worship anything else besides Him." Then he proceeded for a while and then said, "O Muadh bin Jabal!" I replied, "Labbaik, O Allah's Apostle, Sa'daik!' He said, "Do you know what is the right of the slaves upon Allah if they do that?" I replied, "Allah and His Apostle know best." He said, "The right of the slaves upon Allah is that He will not punish them (if they do that)."