Ibrahim (may Allah have mercy on him) said about a man who seals his will and says: "Bear witness to what is in it": He said: It is not permissible until he reads it himself, or it is read to him and he acknowledges what is in it.
(b) Muhammad bin Yusuf (may Allah have mercy on him) said that Sufyan (may Allah have mercy on him) was asked about a man who writes a will, then seals it and says: "Bear witness to whatever is in it." Ibn Abi Layla (may Allah have mercy on him) considered it invalid, and the judge did not consider it permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20420