There doesn't appear to be a completely authentic source. It's cited to be part of a 1988 graduation speech at Bristol University, and appears to be more correctly "live modestly and do serious things". As far as I can tell, no printed copy of that speech exists, so it's all hearsay.

The closest I can get to an authentic citation is a quote of it by a colleague in an obituary published by the Royal Society[1]. It rings authentic, in that it seems like something that would be said in the context of a graduation speech, and as such it may not have left a written record.

[1] https://vdocuments.site/dorothy-mary-crowfoot-hodgkin-om-12-...