How is testimony based on hearing (speech) categorized and ruled upon?

Primary text

Testimony based on hearing is of two types. The first type involves hearing the words of the party being testified against, such as in contracts like sale or lease agreements. In such cases, hearing the words of the contracting parties is necessary, but seeing them is not required, provided one knows the speakers and is certain that the speech belongs to them. This view is held by Ibn Abbas, Al-Zuhri, Rabi'ah, Al-Layth, Shurayh, 'Ata', Ibn Abi Layla, and Malik. Our evidence is that if the party being testified against is certainly known, testimony is permissible, just as if one had seen him. The possibility of confusion among voices is comparable to the possibility of confusion among images. Testimony is only valid for one who certainly knows the party being testified against, and certainty can be attained through hearing, as evidenced by the Law permitting narration without sight, and by accepting the narration of a blind person or one who narrated from the Prophet's wives without being their *mahram* (unmarriageable kin).