Musannaf Ibn Abi Shayba 22:365

Mu'awiyah ibn Hisham narrated to us, he said: Al-Muradi narrated to us, from 'Amr ibn Maysarah, from Abu 'Abd al-Rahman al-Sulami, he said: "If you differ regarding the Qur'an in a 'ya' or a 'ta', then make it a 'ya', for the Qur'an was revealed upon the 'ya'."