Open mouth

Jesus sits on a mountain and opens His mouth to teach (Matthew 5:2).  The phrasing is unusual; I have found only one place in the OT  where opening the mouth is linked with teaching – Proverbs 31, 26, where it is the excellent woman who opens her mouth with wisdom and teaches kindness.

Earlier in Proverbs 31, the phrase is used a couple of times, not in connection with teaching but in connection with royal judgment.  Kings ought not open their mouths to drink wine and strong drink (v. 4), but they should instead open their mouths to be the mouthpiece for the dumb, to defend the righteous of the afflicted and needy (vv. 8-9).

Which is just what Jesus proceeds to do: As the King on the mountain, He pronounces blessings on the afflicted.