From Sahih Bukhari (Hadith 2856):
Narrated Mu`adh:I was a companion rider of the Prophet (ﷺ) on a donkey called 'Ufair. The Prophet (ﷺ) asked, "O Mu`adh! Do you know what Allah's right on His slaves is, and what the right of His slaves on Him is?" I replied, "Allah and His Apostle know better." He said, "Allah's right on His slaves is that they should worship Him (Alone) and should not worship any besides Him. And slave's right on Allah is that He should not punish him who worships none besides Him." I said, "O Allah's Messenger (ﷺ)! Should I not inform the people of this good news?" He said, "Do not inform them of it, lest they should depend on it (absolutely)
Reference: Bukhari 2856