Yunus narrates from al-Hasan (may Allah have mercy on him) that he disliked the testimony of a blind man.
Imam al-Shafi'i (may Allah have mercy on him) said: If this is so, then a written document is even more fitting (of the ruling) that it is not lawful for anyone to testify on the basis of it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20587