俱舍論記 28.124

28.124

「有餘師說至照而無動」者，有餘師說：第四靜慮如密室燈照而無動，故名不動。喻經說故。

"Other masters say [...] shines without flickering": other masters say: the fourth dhyāna is like a lamp in a closed room that shines without flickering, and is therefore called immovable. This is because it is spoken in a metaphorical sūtra.