The greatest commandment is named by Jesus in Matthew 22:37-40, with a parallel account in Mark 12:29-31. In the King James Version it reads:
Matthew 22:37-40 (KJV)
37 Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. 38 This is the first and great commandment. 39 And the second is like unto it, Thou shalt love thy neighbour as thyself. 40 On these two commandments hang all the law and the prophets.
Mark 12:29-31 (KJV)
29 And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord: 30 And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment. 31 And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these.