大乘起信論義記 3.267

3.267

次釋理無，本以待色為空，今既唯心無色，何得更有於空？故云以一切色法乃至虛空之相也。

Next, explaining that [space is] non-existent in principle: originally, [space] was considered void by contrast with matter; now, since there is only mind and no matter, how could there still be space? Therefore [the text] says: "Because all material phenomena [...] the mark of empty space."