For ye know what commandments - It was but a short time since Paul was with them, and they could not but recollect the rules of living which he had laid down.
By the Lord Jesus - By the authority of the Lord Jesus. Some of those rules, or commandments, the apostle refers to, probably, in the following verses.

Ye know what commandments we gave you - This refers to his instructions while he was among them; and to instructions on particular subjects, which he does not recapitulate, but only hints at.

For ye know what commandments we gave you,.... When among them; such as those of faith and love, the ordinances of the Gospel, baptism, and the Lord's supper, and all such as relate to the worship and service of God, to the discipline of Christ's house, to their behaviour one towards another, and their conduct in the world: and which were delivered to them, not as from themselves, and by their own authority, but
by the Lord Jesus; in his name, and by his authority, and as ordered by him; for their commission ran to teach men all things, whatsoever Christ commanded: now since they knew what these commandments were, and whose they were, and the obligation they lay under to regard them, the apostle makes use of it as a reason or argument to engage them to obedience to them; for he that knows his Lord's will, and does it not, shall be beaten with many stripes, Luke 12:47.

by the Lord Jesus--by His authority and direction, not by our own. He uses the strong term, "commandments," in writing to this Church not long founded, knowing that they would take it in a right spirit, and feeling it desirable that they should understand he spake with divine authority. He seldom uses the term in writing subsequently, when his authority was established, to other churches. 1-Corinthians 7:10; 1-Corinthians 11:17; and 1-Timothy 1:5 (1-Thessalonians 4:18, where the subject accounts for the strong expression) are the exceptions. "The Lord" marks His paramount authority, requiring implicit obedience.