Musannaf Ibn Abi Shayba 3:6162

'Abd al-Rahman ibn Muhammad al-Muharibi narrated to us, from Layth, from Mujahid, that he used to dislike that one should write the tenth-part markers (ta'shir) in the Mushaf, or separate Surah al-Baqarah, or say "the Surah in which al-Baqarah is mentioned."