Oneness, Uniqueness of Allah (Tawheed)

Bukhari :: Book 9 :: Volume 93 :: Hadith 470

Narrated Mu'adh bin Jabal:

The Prophet said, "O Mu'adh! Do you know what Allah's Right upon His slaves is?" I said, "Allah and His Apostle know best." The Prophet said, "To worship Him (Allah) Alone and to join none in worship with Him (Allah). Do you know what their right upon Him is?" I replied, "Allah and His Apostle know best." The Prophet said, "Not to punish them (if they do so)."