Original: According to Vasari, Leonardo collaborated with Verrocchio on his Baptism of Christ, painting the young angel holding Jesus's robe in a manner that was so far superior to his master's that Verrocchio put down his brush and never painted again.
Simplified: He gave Leonardo the job of painting one of the angels holding Jesus' robe on the left side of the picture. Vasari said that Leonardo painted the angel so beautifully that Verrocchio put down his brush and never painted again.