It is in Applied Optimal Control by Bryson and Ho (1969). Yann LeCun acknowledges this in his 1989 paper on backpropagation:https://new.math.uiuc.edu/MathMLseminar/seminarPapers/LeCunB....

> "Since his first work on the subject, the author has found that A. Bryson and Y.-C. Ho [Bryson and Ho, 1969] described the backpropagation algorithm using Lagrange formalism. Although their description was, of course, within the framework of optimal control rather than machine learning, the resulting procedure is identical to backpropagation."