Question: If the deceased wrote in a notebook what he owed and what was owed to him—debts, injustices, claims, and deposits—clearly laying it out in his own handwriting, known to his heirs and others, but did not have witnesses—must his heirs and executor carry that out, or not?
Answer—and with Allah is success: If the heirs and the executor verify and recognize the handwriting, then it is obligatory upon them to carry out what is in the deceased’s written statement. What the deceased set down in his own hand counts as an acknowledgment of its contents and a confession to what it comprises.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1