Jesus never said a single thing about slavery as far as I know. But slavery is mentioned in other books in the New Testament and there is no doubt from these books that some Christians owned slaves:
Colossians 3:22: Slaves, obey your earthly masters in everything, not only while being watched and in order to please them, but wholeheartedly, fearing the Lord
Ephesians 6:5-6: Slaves, obey your earthly masters with fear and trembling, in singleness of heart, as you obey Christ; not only while being watched, and in order to please them, but as slaves of Christ, doing the will of God from the heart.
1 Timothy 6:1-2: Let all who are under the yoke of slavery regard their masters as worthy of all honor, so that the name of God and the teaching may not be blasphemed.2 Those who have believing masters must not be disrespectful to them on the ground that they are members of the church;[a] rather they must serve them all the more, since those who benefit by their service are believers and beloved.
Titus 2:9-10: Tell slaves to be submissive to their masters and to give satisfaction in every respect; they are not to talk back, 10 not to pilfer, but to show complete and perfect fidelity, so that in everything they may be an ornament to the doctrine of God our Savior.
1 Peter 2:18-21: 18 Slaves, accept the authority of your masters with all deference, not only those who are kind and gentle but also those who are harsh. 19 For it is a credit to you if, being aware of God, you endure pain while suffering unjustly. 20 If you endure when you are beaten for doing wrong, what credit is that? But if you endure when you do right and suffer for it, you have God’s approval. 21 For to this you have been called, because Christ also suffered for you, leaving you an example, so that you should follow in his steps.
As this last passage from Peter says, some have been called by God to be slaves.