It's taken from John 9:1-20. It's a great miracle that Jesus performed to show that he was the son of God.

Jesus heals a man who was born blind. John 9:1-20

As Jesus went on His way, He saw a man who had been born blind. His followers asked Him, "Teacher, whose sin made this man to be born blind? Was it the sin of this man or the sin of his parents?" "The sin of this man or the sin of his parents did not make him to be born blind. He was born blind so the work of God would be seen in him. We must keep on doing the work of Him Who sent Me while it is day. Night is coming when no man can work. While I am in the world, I am the Light of the world."

After Jesus had said this, He spit on the ground. He mixed it with dust and put that mud on the eyes of the blind man. "Go and wash in the pool of the Siloam."

“While I am in the world, I am the Light of the world."