Source identified1
“Humility—to put a seal upon your lips and forget what you have done.”
Drummond defines humility as allowing good work to remain larger than the ego that performed it: kindness is followed by a refusal to turn the act into self-advertisement or private self-congratulation.
Source note: “The Greatest Thing in the World,” in The Greatest Thing in the World and Other Addresses, Project Gutenberg #16739. Drummond explicitly places Humility among the “ingredients” of Christian love and glosses it as love that does not vaunt itself.