Mark 12:28-29 (NIV)
The Greatest Commandment
28 One of the teachers of the law came and heard them debating. Noticing that Jesus had given them a good answer, he asked him, “Of all the commandments, which is the most important?”29 “The most important one,” answered Jesus, “is this: ‘Hear, O Israel: The Lord our God, the Lord is one.
Deuteronomy 6:4 (NIV)
4 Hear, O Israel: The Lord our God, the Lord is one.