Musannaf Ibn Abi Shayba 22:333

Al-Muharibi narrated to us, from Layth, from Mujahid, that he used to dislike writing the "ta'shir" (marking every ten verses) or the "tafsil" (division/details) [in the Mushaf]. He would say: "Surat al-Baqarah." And he would say: "The Surah in which the cow (al-baqarah) is mentioned."