Riyad as-Salihin 1711 — The Book of the Prohibited actions

Ibn 'Umar Narrated from:

I heard a man saying: "No, by the Ka'bah." I admonished him: "Do not swear by any thing besides Allah, for I heard the Messenger of Allah (ﷺ) saying, 'He who swears by anyone or anything other than Allah, has indeed committed an act of Kufr or Shirk'." [At- Tirmidhi]. [Some 'Ulama' are of the opinion that the words of the Prophet (ﷺ) that "He who swears by anyone or anything other than Allah has indeed committed an act of Kufr or Shirk," are in the nature of extreme admonition. And in fact, it is not Shirk. The same applies to the saying of the Prophet (ﷺ), who said, "showing off is Shirk."]