Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon LXXXV

Canon LXXXV.

We have received from the Scriptures that in the mouth of two or three witnesses every word shall be established.  Therefore we decree that slaves who are manumitted by their masters in the presence of three witnesses shall enjoy that honour; for they being present at the time will add strength and stability to the liberty given, and they will bring it to pass that faith will be kept in those things which they now witness were done in their presence.

Ancient Epitome of Canon LXXXV.

A slave manumitted by his master before two witnesses shall be free.