Musannaf Ibn Abi Shayba 18:187

Abu Bakr narrated to us, he said: Humayd ibn Abd al-Rahman narrated to us, from Hasan, from Bayan, from al-Sha'bi, who said: The statement of Ali was mentioned regarding the one whose eye was injured, where he showed him the egg (as a test). Al-Sha'bi said: If he wishes, he may increase [the vision] in the eye with which he sees, and he said: He will see with it more than he used to see with it. And if he wishes, he may decrease [the vision] in the eye that was injured, and he said: He will not see with it, and he does not see with it. But the best way for that is for a physician to look at what he sees, and he should look at what has been diminished from it.