How the Christ of God appeared to him in his Sleep, and commanded him to use in his Wars a Standard made in the Form of the Cross
He said, moreover, that he doubted within himself what the import of this apparition could be. And while he continued to ponder and reason on its meaning, night suddenly came on; then in his sleep the Christ of God appeared to him with the same sign which he had seen in the heavens, and commanded him to make a likeness of that sign which he had seen in the heavens, and to use it as a safeguard in all engagements with his enemies.
Source: Life of Constantine (New Advent)