But you shall receive power, when the Holy Spirit has come upon you: and you shall be my witnesses in Jerusalem, and in all Judea, and in Samaria, even to the uttermost part of the earth. (Jesus, Acts 1:8)
Many of the things Jesus said contained allusions to the earlier writings of the Old Testament.
You are my witnesses, says the Lord, and my servant whom I have chosen: that you may know and believe me, and understand that I am he: before me there was no God formed, neither shall there be after me. (Isaiah 43:10)Are there other references in the Bible to being witnesses? Sure; they generally involve legal matters or transactions. The clearest parallel to what Jesus said are the passages from Isaiah quoted above. Jesus' apostles, all Jewish, were likely to have caught the references. Once again, Jesus is recorded as saying something that parallels what God himself has said, reprising one of God's sayings in a way that makes Jesus' role parallel to God's own role.
I have declared, and have saved, and I have shown, when there was no strange god among you: therefore you are my witnesses, says the LORD, that I am God. (Isaiah 43:12)
Do not fear, neither be afraid: Have I not told you from that time, and declared it? You are my witnesses. Is there a God beside me? There is no other God; I know not any. (Isaiah 44:8)
One other point bears mentioning: God chose the language of "witnesses" for us. In this world, sometimes God is on trial, either in the court of public opinion or in an individual's mind and life. People want to know whether God is true, whether God is kind, whether God is trustworthy. They want to know whether he cares. When we notice that God is on trial, or God is accused, it is worth remembering that we are his witnesses.