Thabit bin Dahhak reported,

The Prophet ﷺ said, "Whoever swears by a religion other than Islam, lying intentionally, is as he said. And whoever kills himself with a piece of iron will be tormented with it in the fire of Hell."

Azami 13320

Sahih

عن ثابت بن الضحاك- وكان من أصحاب الشجرة- قال

قال النبي ﷺ: من حلف بملة غير الإسلام كاذبا متعمدا فهو كما قال، ومن قتل نفسه بحديدة عذب به في نار جهنم.

الأعظمي 13320صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Tafsir of the great Quran
Chapter
Tafsir of Surah Fath
Section
His saying, "Allah was certainly pleased with the believers when they pledged allegiance to you under the tree, and He knew what was in their hearts, so He sent down tranquility upon them and rewarded them with a near victory"
Numbering
Jami al-Kamil 13320 (2nd edition)
Azami 13320