The Father witnesses that Jesus is his son

God testified of his son, Jesus Christ, at his baptism. See Matthew 3: 13-17.

Then cometh Jesus from Galilee to Jordan unto John, to be baptized of him.
But John forbad him, saying, I have need to be baptized of thee, and
comest thou to me?
And Jesus answering said unto him, Suffer it to be so now: for thus it
becometh us to fulfill all righteousness. Then he suffered him.
And Jesus, when he was baptized, went up straightway out of the water:
and, lo, the heavens were opened unto him, and he saw the Spirit of
God descending like a dove, and lighting upon him:
And lo a voice from heaven, saying, This is my beloved Son, in whom I
am well pleased.

In addition to the Lord's baptism, in recorded history God, the
Father, has spoken three other times, each time testifying of his son.

On the Mount of Transfiguration recorded in Matt. 17: 5. (Read also
Mark 9: 7 and Luke 9: 35).
While he yet spake, behold, a bright cloud overshadowed them: and
behold a voice out of the cloud, which said, This is my beloved Son,
in whom I am well pleased; hear ye him.

To the Nephite people as recorded in the 3 Nephi 11: 7 in the Book of Mormon.
Behold my Beloved Son, bin whom I am well pleased, in whom I have
glorified my name—hear ye him.

And finally to Joseph Smith as recorded in Joseph Smith's own words:
It no sooner appeared than I found myself delivered from the enemy
which held me bound. When the light rested upon me I saw two
Personages, whose brightness and glory defy all description, standing
above me in the air. One of them spake unto me, calling me by name and
said, pointing to the other—This is My Beloved Son. Hear Him!