This is written about in two books of the Bible.
From Matthew 22:
35 Then one of them, which was a lawyer, asked him a question, tempting him, and saying,
36 Master, which is the great commandment in the law?
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.
And
From Mark 12:
28 And one of the scribes came, and having heard them reasoning together, and perceiving that he had answered them well, asked him, Which is the first commandment of all?
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.