Q 11:
Is it permissible to swear by Allah and His Messenger?
A:
It is impermissible to do so, for the Messenger of Allah (peace be upon him) said,
Whoever makes an oath should swear by Allah or else remain silent. (i.e. He should not swear by anyone other than Allah.)
And
Whoever swears by anyone other than Allah is a disbeliever or a Mushrik (one who associates others with Allah in His Divinity or worship).
(Part No. 23; Page No. 57) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.