Read it in a book by a Spanish doctor. I read it in Danish, but I think the original is called "Duérmete, Niño". It is written by Eduard Estivill and Sylvia De Béjar from what I can find using Google.

(I think people often misunderstand it so you'll find some people saying he argues that kids should cry it out or something, while the idea is rather to make them feel safe and enjoy bedtime.)