The Messenger of Allah ﷺ said, "The believer's lower garment reaches to the muscle of his calves, then to the middle of his shins, then to his ankles. Whatever falls below that is in the Fire."
حدثنا الوليد بن مسلم حدثنا الأوزاعي حدثنا يحيى يعني ابن أبي كثير عن محمد بن إبراهيم التيمي عن يعقوب أو ابن يعقوب
قال رسول الله ﷺ إزرة المؤمن إلى عضلة ساقيه ثم إلى نصف ساقيه ثم إلى كعبيه فما كان أسفل من ذلك في النار
Its isnad is sahih, despite some uncertainty regarding the name of one of its narrators. We have verified this and discussed it in detail at 7460 and 7461. Al-izrah, with the hamzah vowelled as "i", means the manner and style of wearing the waist-wrap, like "rukbah" (manner of kneeling) and "jilsah" (manner of sitting). So said Ibn Athir.