Madhvaon Brahma Sūtra 4.2.18

He departs. For the Pautrayana Sruti says: “A thousand are the rays of the Sun, which are spread through these veins; one of them (rays) which is white and is called ‘ Sushumna/ and is the way leading to Brahman, has penetrated the vein called ‘ Sushutnna/ and by that light he issues out.”