Tauhid (Oneness of Allah)

Belief in the oneness of Allah and expression of it through the tongue is known as Tauhid

For one thing man reason (provided it is sound) affirms that Allah is One. Hence all men of wisdom all great scholars of the world have had belief in the Oneness of Allah
Secondaly the prophets of Allah have unanimously given mankind the creed of the Oneness of Allah and told them that Allah is One and none else is like unto Him

Holy Quran does contain the creed of the Oneness of Allah in an excellent manner and perfect way. The Holy Quran is rather the only Book that preaches the Oneness of Allah in its purest form. The earlier Scriptures too preached the Oneness of Allah but the people made interpolations in them and included facts against the principle of Oneness and altered the Divine teaching. Allah send Prophet Muhammad (PBUH) to bring about reformation in them and preach the creed of Oneness to the world. He revealed His Book, the Holy Quran to the prophet and he preached he true creed of Oneness in every clear terms