隋书
The Göktürk rulers originated from the Ashina clan, who were first attested to in 439. The "Book of Sui" reports that in that year, on October 18, the Tuoba ruler Emperor Taiwu of Northern Wei overthrew Juqu Mujian of the Northern Liang in eastern Gansu, whence 500 Ashina families fled northwest to the Rouran Khaganate in the vicinity of Gaochang.