。
Let sleeping lions lie.
Literal
Sleeping lion [topic-は] wake-prohibitive-な.
眠れる is classical attributive form of 眠る (not the modern potential 'can sleep'). Here it's 'sleeping' as an adjectival modifier. 獅子 ('lion') is a more literary word than ライオン. な is the prohibitive. A classical rendering of the 'let sleeping dogs lie' proverb.