I first encountered similar advice when reading the slides of David Patterson’s talk “How to Have a Bad Career in Research/Academia” as an undergraduate in the late 2000s who was planning a career in computer science research:

https://people.eecs.berkeley.edu/~pattrsn/talks/BadCareer.pd...

It might be Paul Halmos. In his "automathobiography", titled I Want to Be a Mathematician (Springer 1985), page 156: "In the late 1940's I began to act on one of my beliefs: to stay young, you have to change fields every five years". He goes on saying "I didn't first discover it and then act on it, but instead, noting that I did in fact seem to change directions every so often, I made a virtue out of a fact and formulated it as a piece of wisdom".