Verses with the word ambassador in the Old Testament (3 verses):

Proverbs 13:17
A wicked messenger falleth into mischief: but a faithful ambassador is health.

Jeremiah 49:14
I have heard a rumour from the LORD, and an ambassador is sent unto the heathen, saying, Gather ye together, and come against her, and rise up to the battle.

Obadiah 1:1
The vision of Obadiah. Thus saith the Lord GOD concerning Edom; We have heard a rumour from the LORD, and an ambassador is sent among the heathen, Arise ye, and let us rise up against her in battle.