Don’t shine so others can see you. Shine so that through you, others can see Him.
Attributed to C. S. Lewis, but not by him. Someone already did the work on this for me. http://www.essentialcslewis.com/2016/03/05/ccslq-22-dont-shine/
Earliest known attribution to CSL is 2014. It’s not even on the discussion page for Lewis at Wikiquote, so I knew it was mostly likely a recent misattribution.
Additional thought: This sounds an awful lot like something from Cardinal Newman’s meditation on radiating Christ, which is the foundation for the Missionaries of Charity’s prayer discussed here.