Is there scripture showing King Solomon went to Heaven?
The Bible never explicitly states Solomon's destination at death, nor for that matter does the Bible explicitly state the eternal designation of many other saints mentioned in Scripture. Nevertheless, the faith of Solomon (and therefore his salvation) is never in doubt in Scripture.
Several facts confirm his faith for us, including:
1. Solomon is recorded in the line of Messiah, and there are no unbelievers in the line of Messiah.
2. Solomon wrote several books of Scripture (Proverbs, Ecclesiastes, Songs of Solomon), and God does not use an unbeliever to author any book of Scripture.
3. Solomon's own words found in the Bible confirm his faith, including Solomon recording the words of the Second Person of the Godhead (Jesus pre-incarnate):
Prov. 8:22 “The LORD possessed me at the beginning of His way,
Before His works of old.
Prov. 8:23 “From everlasting I was established,
From the beginning, from the earliest times of the earth.
Prov. 8:24 “When there were no depths I was brought forth,
When there were no springs abounding with water.
Prov. 8:25 “Before the mountains were settled,
Before the hills I was brought forth;
Prov. 8:26 While He had not yet made the earth and the fields,
Nor the first dust of the world.
Prov. 8:27 “When He established the heavens, I was there,
When He inscribed a circle on the face of the deep,
Prov. 8:28 When He made firm the skies above,
When the springs of the deep became fixed,
Prov. 8:29 When He set for the sea its boundary
So that the water would not transgress His command,
When He marked out the foundations of the earth;
Prov. 8:30 Then I was beside Him, as a master workman;
And I was daily His delight,
Rejoicing always before Him,
Prov. 8:31 Rejoicing in the world, His earth,
And having my delight in the sons of men.
Prov. 8:32 “Now therefore, O sons, listen to me,
For blessed are they who keep my ways.
Prov. 8:33 “Heed instruction and be wise,
And do not neglect it.
Prov. 8:34 “Blessed is the man who listens to me,
Watching daily at my gates,
Waiting at my doorposts.
Prov. 8:35 “For he who finds me finds life
And obtains favor from the LORD.
Prov. 8:36 “But he who sins against me injures himself;
All those who hate me love death.”
Prov. 9:9 Give instruction to a wise man and he will be still wiser,
Teach a righteous man and he will increase his learning.
Prov. 9:10 The fear of the LORD is the beginning of wisdom,
And the knowledge of the Holy One is understanding.
There are many more examples of Solomon's writing of the Messiah to come, but these examples re enough to prove the point. Solomon was a man saved by faith in Messiah, who declared in Scripture under the inspiration of the Holy Spirit.
Solomon's life was a mixed testimony, which included both commendable and regrettable actions, but this is common for all believers. Every believer is saved by grace through faith in Jesus while living a life that includes both righteous and unrighteous acts. If we judge Solomon's sin to be worse than our own, then we are imagining ourselves as more deserving of God's grace, and that is self-righteousness.