No particular source (just like general knowledge now), but it’s easy to find. Here’s a reddit thread discussing it and some links. https://www.reddit.com/r/evolution/comments/18sbbbh/what_percentage_of_men_actually_reproduced/
The reddit OP uses 33% not 40% for men, and something I didn’t know about that is like way more extreme:
Once upon a time, 4,000 to 8,000 years after humanity invented agriculture, something very strange happened to human reproduction. Across the globe, for every 17 women who were reproducing, passing on genes that are still around today—only one man did the same.