Bava Batra 153 - Was He Sick When Writing a Will?
A sickbed will ordinarily states that the donor wrote it "when he was sick and lying in bed," whereas the ordinary gift document generally specifies "while healthy".
Someone wrote a gift document but did not record the fact that he was a seriously ill person, nor that he was healthy, and then a dispute arouse about the validity of the gift. The donor says "I was sick when I wrote the document and I have since recovered, hence my recovery nullifies the gift," but the recipients say, "You were healthy, and the gift is therefore valid."
The giver must bring proof that he was seriously ill - these are the words of Rabbi Meir, but the Sages say that since recipients wish to collect from the giver, who is presently in the possession of the property, they are the ones who must prove their claim.
Art: H. Lessing - The Sick Bed