Musar zu Jeschijahu 57:5

And one who plays with children and [masturbates] whether with the hand or the foot is liable. And our Rabbis, may their memory be blessed, said (Niddah 13b) that his punishment is like the punishment of the generation of the flood, that had corrupted their ways. And likewise one who does like the act of Er and Onan - to thresh (the penis) inside (the vagina) and winnow outside, to destroy the seed - is liable, as it is stated (Genesis 38:10), “And what he did was displeasing to the Lord, and He killed him as well.” And it is stated about those that waste seed (Isaiah 57:5), “You who inflame yourselves among the terebinths, under every verdant tree; who slaughter children, etc.”