Ibn Umar told that he heard God’s Messenger say, “He who swears by anyone but God is a polytheist.” Tirmidhi transmitted it.

وَعَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «مَنْ حَلَفَ بِغَيْرِ اللَّهِ فَقَدْ أَشْرَكَ» . رَوَاهُ التِّرْمِذِيّ