In waṣiyyah bi-l-kitāb, a teacher directs that his original manuscripts or books be transferred to a particular person when he dies or travels. This may establish custody of the material without automatically establishing permission to transmit it.
Some early scholars accepted a broader form of narration based on such a bequest, but the majority did not treat a bare waṣiyyah as sufficient without separate permission. Ownership, custody, and authorization have different evidentiary implications.
Example: A hadith scholar tells his son, “Keep these notebooks after my death.” The statement may establish lawful custody; it does not necessarily authorize the son to claim a direct transmission of every report written in those notebooks.