So the Pharisees and Saducees were always trying to trick Jesus, as if they would know more than the Son of God. They always thought they had some new trick question. So here they come up to him and ask what is the greatest commandment, something I am sure they debated all the time, just as we debate what is the greatest sin all the time.

But when the Pharisees heard that He had silenced the Sadducees, they gathered together.
Then one of them, a lawyer, asked Him a question, testing Him, and saying,
“Teacher, which is the great commandment in the law?”
Jesus said to him, ‘YOU SHALL LOVE THE LORD YOUR GOD WITH ALL YOUR HEART, WITH ALL YOUR SOUL, AND WITH ALL YOUR MIND.’
This is the first and great commandment.
And the second is like it: ‘YOU SHALL LOVE YOUR NEIGHBOR AS YOURSELF.’
On these two commandments hang all the Law and the Prophets.”
Matthew 22:34-40 NKJV

All this comes from the old testament, most referenced are these:

“Hear, O Israel: The LORD our God, the LORD is one!
You shall love the LORD your God with all your heart, with all your soul, and with all your strength.
Deuteronomy 6:4-5 NKJV

You shall not take vengeance, nor bear any grudge against the children of your people, but you shall love your neighbor as yourself: I am the LORD.
Leviticus 19:18 NKJV

So Jesus not only comes back with the greatest but also the second greatest, a two for one special as it were.

So were does this come from? The verses in Deut & Lev but lets also take a look at the Ten commandments and see how they break down. If you look at the first four they relate to Jesus answer for the greatest commandment. If you look at the last six they cover the second greatest commandment.

And God spoke all these words, saying: “I am the LORD your God, who brought you out of the land of Egypt, out of the house of bondage.
Exodus 20:1-2 NKJV

1
“You shall have no other gods before Me.
Exodus 20:3 NKJV
2
“You shall not make for yourself a carved image–any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth; you shall not bow down to them nor serve them. For I, the LORD your God, am a jealous God, visiting the iniquity of the fathers upon the children to the third and fourth generations of those who hate Me, but showing mercy to thousands, to those who love Me and keep My commandments.
Exodus 20:4-6 NKJV
3
“You shall not take the name of the LORD your God in vain, for the LORD will not hold him guiltless who takes His name in vain.
Exodus 20:7 NKJV
4
“Remember the Sabbath day, to keep it holy.
Six days you shall labor and do all your work, but the seventh day is the Sabbath of the LORD your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who is within your gates.
For in six days the LORD made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the LORD blessed the Sabbath day and hallowed it.
Exodus 20:8-11 NKJV

5
“Honor your father and your mother, that your days may be long upon the land which the LORD your God is giving you.
Exodus 20:12 NKJV
6
“You shall not murder.
Exodus 20:13 NKJV
7
“You shall not commit adultery.
Exodus 20:14 NKJV
8
“You shall not steal.
Exodus 20:15 NKJV
9
“You shall not bear false witness against your neighbor.
Exodus 20:16 NKJV
10
“You shall not covet your neighbor’s house; you shall not covet your neighbor’s wife, nor his male servant, nor his female servant, nor his ox, nor his donkey, nor anything that is your neighbor’s.”
Exodus 20:17 NKJV

So in what ever you are doing: Love God, Love Others!