What is the ruling on swearing by the Prophet, may God’s prayers and peace be upon him?

It is not permissible to swear by the Prophet, peace and blessings be upon him, because he said, "Whoever takes an oath, let him swear by Allah or remain silent." The saying that swearing by him necessitates expiation upon breaking the oath is a weak saying and contrary to the evidence, because it is an oath by other than Allah, and thus does not necessitate expiation.