Matthew 1:20

But after he had pondered these things, an angel of the Lord appeared to him in a dream and said, “Joseph, son of David, do not be afraid to embrace Mary as your wife, for the One conceived in her is from the Holy Spirit.
BSB · Berean Standard Bible
But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost.
KJV · King James Version
But when he thought about these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, don’t be afraid to take to yourself Mary as your wife, for that which is conceived in her is of the Holy Spirit.

Note “Behold”, from “ἰδοὺ”, means look at, take notice, observe, see, or gaze at. It is often used as an interjection.

WEB · World English Bible
But when he thought on these things, behold, an angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Spirit.
ASV · American Standard Version

Related passages

From the Treasury of Scripture Knowledge, grouped by the word they explain.

while
Psalm 25:8, 9; Psalm 94:19; Psalm 119:125; Psalm 143:8; Proverbs 3:5, 6; Proverbs 12:5; Isaiah 26:3; Isaiah 30:21
the angel
Judges 13:3, 8, 9; Luke 1:10–13, 19, 26–38; Luke 2:8–14
in
Matthew 2:13, 19, 22; Genesis 31:11; Numbers 12:6; Job 4:13–16; Job 33:15–17; Joel 2:28
Joseph
Isaiah 7:2, 13; Jeremiah 33:26; Luke 2:4
fear not
Matthew 28:5; Genesis 46:3; 1 Kings 17:13; Isaiah 51:7; Jeremiah 40:9; Luke 1:30
that
Matthew 1:18; Jeremiah 31:22