2 Thessalonians 2:15

15 So then, brothers and sisters, stand firm and hold fast to the teachingsa we passed on to you, whether by word of mouth or by letter.

Read 2 Thessalonians 2:15 Using Other Translations

Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
So then, brothers, stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter.
With all these things in mind, dear brothers and sisters, stand firm and keep a strong grip on the teaching we passed on to you both in person and by letter.