In 1466, at the age of fourteen, Leonardo was apprenticed to the artist Andrea di Cione, known as Verrocchio, whose bottega (workshop) was "one of the finest in Florence". He apprenticed as a garzone (studio boy) to Andrea del Verrocchio, the leading Florentine painter and sculptor of his day (and would do so for 7 years). Other famous painters apprenticed or associated with the workshop include Domenico Ghirlandaio, Perugino, Botticelli, and Lorenzo di Credi. Leonardo would have been exposed to both theoretical training and a vast range of technical skills, including drafting, chemistry, metallurgy, metal working, plaster casting, leather working, mechanics and carpentry as well as the artistic skills of drawing, painting, sculpting and modelling.  Much of the painted production of Verrocchio's workshop was done by his employees. According to Vasari, Leonardo collaborated with Verrocchio on his The Baptism of Christ, painting the young angel holding Jesus' robe in a manner that was so far superior to his master's that Verrocchio put down his brush and never painted again, although this is believed to be apocryphal. Close examination reveals areas that have been painted or touched-up over the tempera using the new technique of oil paint; the landscape, the rocks seen through the brown mountain stream and much of the figure of Jesus bearing witness to the hand of Leonardo. Leonardo may have been the model for two works by Verrocchio: the bronze statue of David in the Bargello and the Archangel Raphael in Tobias and the Angel.  By 1472, at the age of twenty, Leonardo qualified as a master in the Guild of Saint Luke, the guild of artists and doctors of medicine, but even after his father set him up in his own workshop, his attachment to Verrocchio was such that he continued to collaborate with him. Leonardo's earliest known dated work is a drawing in pen and ink of the Arno valley, drawn on 5 August 1473.

Answer this question "What was his first painting" by extracting the answer from the text above.
Verrocchio on his The Baptism of Christ,