The most convenient source of information on homogeneous functions is Wikipedia; if that is not satisfactory as a citation, you could cite Apostol's calculus text book (volume 2).

The most convenient source of information on homogeneous functions is Wikipedia; if that is not satisfactory as a source, you could cite:

J.P. Lewis, Homogeneous Functions and Euler’s Theorem. In: An Introduction to Mathematics (Palgrave Macmillan, London, 1969).