Say (O Muhammad): "O people of the Scripture (Jews and Christians): Come to a word that is just between us and you, that we worship none but Allah, and that we associate no partners with Him, and that none of us shall take others as lords besides Allah. Then, if they turn away, say: "Bear witness that we are Muslims." (Aali Imran 3:64)  

Oneness of God (Tawheed) means devoting worship to him alone like supplication, vow, judgment.

Evidence from the Noble Qur'an
"So know (O Muhammad) that there is no God except Allaah" (Muhammad 47:19)

Evidence from the authentic Sunnah (Prophetic statements, actions or approvals of Muhammad - may Allaah raise his rank and grant him peace)
"Let the first thing you invite them to do is to testify that none has the right to be worshipped but Allaah" [Transmitted by al-Bukhaaree and Muslim]

View the sub-menu to the left to study about Allah and the Oneness of God