Is saying "By the Prophet, I will do" or "By the Prophet, do not do" considered swearing by other than Allah and an act of polytheism (shirk)? And is the saying of the Prophet, peace and blessings be upon him, "By my life, he is worthy of leadership" considered an oath? And did Imam Ahmad ibn Hanbal permit swearing by the Prophet, peace and blessings be upon him, and what is his evidence for that?

Swearing by other than Allah is polytheism (shirk), based on the Prophet's (peace be upon him) saying: "Whoever swears by other than Allah has disbelieved or committed shirk." Therefore, it is not permissible to swear by the Prophet (peace be upon him). What is attributed to Imam Ahmad concerning the permissibility of swearing by the Prophet is incorrect. Rather, some Hanbalis said that swearing by him constitutes a valid oath requiring expiation, but scholars have deemed this opinion weak. As for the saying "By my life" (laamri), scholars have differed on its permissibility; Malik disliked it, but it has been authentically reported from the Prophet (peace be upon him) and some Companions.