Beloved, I am not writing you a new command, but an old command that you have had from the beginning. This old command is the word you have heard.
In context
5But whoever keeps his word truly has God’s love made complete in him. This is how we know we are in him. 6Whoever claims to remain in him ought to walk just as he walked. 7Beloved, I am not writing you a new command, but an old command that you have had from the beginning. This old command is the word you have heard. 8Yet I am writing you a new command, which is true in him and in you, because the darkness is passing away, and the true light is already shining. 9Whoever claims to be in the light but hates his brother or sister is still in the darkness.