God is one, not two or three

the oneness of God in the Old Testament is found in Deuteronomy chapter 6 verse 4
"Hear, O Israel: The Lord our God, the Lord is one."

Another relevant verse is Isaiah 45:5,
This verse is part of the Shema, a foundational declaration of monotheism in Judaism. While it emphasizes the absolute oneness of God, rejecting the idea of multiple gods.
"I am the Lord, and there is no other; apart from me there is no God."

The quote "God is one, not two or three" can be found in the Qur'an, specifically in Surah Al-Nisa (Chapter 4, Verse 171). The verse emphasizes the oneness of God in Islam