Is that even a commandment?
Yes. Jesus is specifically asked to say "which is the great commandment
in the law?" and replies "Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself." (Matthew 22:37-39.) Both the commandments he cites appear in the Old Testament.
"And thou shalt love the LORD thy God with all thine heart, and with all thy soul, and with all thy might" is Deuteronomy 6:5
"Thou shalt love thy neighbour as thyself" is in Leviticus 19:18