Musannaf Ibn Abi Shayba 18:185

Abu Bakr narrated to us, he said: Abbad bin al-Awwam narrated to us, from Umar bin Amir, from Qatadah, from Sa’id bin al-Musayyib: That a man injured the eye of another man, so some of his vision was lost and some remained. This was brought before Ali, so he ordered that his healthy eye be bandaged. He then ordered a man to take an eggshell and walk away with it while the [injured] man was looking, until his vision reached its limit. He then drew a mark at that point. He said: Then he looked into that and found it to be equal. He said: "Give him [compensation] from the wealth of the other [man] according to the amount that was lost from his vision."