Riyad as-Salihin 1711

Translation

Ibn 'Umar (May Allah be pleased with them) said: 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 (PBUH) 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 (PBUH) 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 (PBUH), who said, "showing off is Shirk."]

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Same hadith, other wordings

• Jami at-Tirmidzi 1535أَنَّ ابْنَ عُمَرَ سَمِعَ رَجُلاً يَقُولُ لاَ وَالْكَعْبَةِ فَقَالَ ابْنُ عُمَرَ لاَ يُحْلَفُ بِغَيْرِ اللَّهِ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ مَنْ حَلَفَ بِغَيْرِ اللَّهِ فَقَدْ كَفَرَ أَوْ أَشْرَكَ
• Misykatul Mashabih 3419وَعَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «مَنْ حَلَفَ بِغَيْرِ اللَّهِ فَقَدْ أَشْرَكَ». رَوَاهُ التِّرْمِذِيّ