First found in 《
西游记》 chapter 99, where the line goes
真人不露相,
露相不真人 (zhēnrén bú lòu xiàng, lòu xiàng bù zhēnrén) — 'a true adept doesn't reveal his true form; one who reveals his form is no true adept'. In Daoist usage
真人 meant a realized immortal; in modern speech it has broadened to anyone genuinely skilled.