Sahih Muslim Book - 1, Hadith - 302

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا مُعَاوِيَةُ بْنُ سَلَّامِ بْنِ أَبِي سَلَّامٍ الدِّمَشْقِيُّ عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ أَنَّ أَبَا قِلَابَةَ أَخْبَرَهُ أَنَّ ثَابِتَ بْنَ الضَّحَّاكِ أَخْبَرَهُ أَنَّهُ بَايَعَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ تَحْتَ الشَّجَرَةِ وَأَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ مَنْ حَلَفَ عَلَى يَمِينٍ بِمِلَّةٍ غَيْرِ الْإِسْلَامِ كَاذِبًا فَهُوَ كَمَا قَالَ وَمَنْ قَتَلَ نَفْسَهُ بِشَيْءٍ عُذِّبَ بِهِ يَوْمَ الْقِيَامَةِ وَلَيْسَ عَلَى رَجُلٍ نَذْرٌ فِي شَيْءٍ لَا يَمْلِكُهُ.

Thabit bin Adh-Dhahhak narrated that he swore allegiance to the Messenger of Allah (s.a.w) beneath the tree, and the Messenger of Allah (s.a.w) said: "Whoever swears falsely that he belongs to a religion (Millat) other than Islam, he is as he said; whoever kills himself with something, he will be punished with it on the Day of Resurrection; and no man is bound by a vow concerning something that he does not possess."