Question: Is it permissible to swear by the Prophet Muhammad (peace be upon him)?
Answer: It is not permissible to swear by any creature, including the Prophet (peace be upon him), the Ka`bah, honesty, or anything else according to the majority of scholars. Rather, some scholars reported a consensus that this is prohibited. An odd view, however, is reported about the permissibility of swearing by the Prophet (peace be upon him), which is a groundless opinion. In fact, it is an invalid view that contradicts the preceding consensus of scholars and authentic Hadeeths mentioned on this issue. Among these Hadeeths is the one related by Al-Bukhaaree and Muslim on the authority of Ameer Al-Mu’mineen (Commander of the Believers), `Umar ibn Al-Khattaab (may Allaah be pleased with him), that the Prophet (peace be upon him) said: Verily! Allaah forbids you to swear by your fathers. If one has to take an oath, they should swear by Allaah or else keep silent. It is also related in the Two Saheeh Books of Hadeeth (i.e. Al-Bukhaaree and Muslim) on the authority of Aboo Hurayrah (may Allah be pleased with him) that the Prophet (peace be upon him) said: Whoever swears, saying (by error) in their oath ‘By Al-Laat and Al-`Uzzaa,’ they should say, ‘Laa ilaahaa illaa Allaah (there is no god but Allaah).’… read more here.
Question 313: When we buy goods from a merchant, we ask him about the price which he mentions. After that, to know the final price, we say: “How much is it for the sake of honesty?” or “How much is it for the sake of Allaah?” What is the ruling on using these or similar phrases?
Answer: It is not permissible to swear by other than Allaah, whether by honesty or anything else, in compliance with the warning of the Prophet (peace be upon him): Anyone who swears by other than Allaah has committed Shirk (associating others with Allaah in His Divinity or worship).... read more here.
Question 309: Is it true that swearing by other than Allaah, using abstract things, is not considered Shirk (associating others with Allaah in His Divinity or worship), whereas swearing by concrete things is that which is considered Shirk?
Answer: Swearing by anything or anyone other than Allaah is an act of minor Shirk (associating other partners with Allaah that is not tantamount to taking a Muslim out of Islaam); based on the Hadeeth reported by Imaam Ahmad, ‘Aboo Daawood and Al-Tirmidee that the Prophet (peace be upon him) said: Anyone who swears by other than Allaah has committed Kufr or Shirk. If a person aims at glorifying the thing sworn by in the same way that Allaah is glorified, this is considered major Shirk (associating others with Allaah in His Divinity or worship that takes the Muslim out of Islaam), whether the thing sworn by is concrete or abstract; all this is not permissible... read more here.