Al-Hasan (may Allah have mercy on him) used to dislike a man testifying to a will written in a sealed document until it is known what is in it.
(b) Abu Qilabah (may Allah have mercy on him) also disliked testifying to what is enclosed in a sealed document, lest there be some injustice in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20419