Oneness, Uniqueness of Allah (Tawheed)

Bukhari :: Book 9 :: Volume 93 :: Hadith 553

Narrated Ibn 'Abbas:

The Prophet stood before Musailama (the liar) who was sitting with his companions then, and said to him, "If you ask me for this piece (of palm-leaf stalk), even then I would not give it to you. You cannot avoid what Allah has ordained for you, and if you turn away from Islam, Allah will surely ruin you! "