Is bearing witness a condition for the validity of revocation, or merely a Sunnah (recommended practice)?

Primary text

Scholars are unanimously agreed that bearing witness during revocation is the Sunnah (recommended practice). If it is held to be a condition, then its presence at the time of revocation is considered. If the husband revokes without witnesses, the revocation is invalid because the existence of witnesses during the act of revocation is what is considered, not subsequent acknowledgement of it. However, if the intention behind the subsequent acknowledgement is to effect the revocation itself, then it becomes valid.