Do you mean that you're looking for the original Japanese phrase in 五輪書 空之巻[Gorin no sho Kuu no maki] corresponding to that? If so, That's 空を道とし道を空と見る所也.
And the sentence containing that phrase is 其心を知て直に成る所を本とし、實の心を道として兵法 を廣く行ひ、正敷明に大き成る所を思ひ取て、空を道と し道を空と見る所也.