Umama reported that the companions of the Messenger of Allah ﷺ were one day discussing worldly matters in his presence, so the Messenger of Allah ﷺ said, "Do you not hear? Do you not hear? Simplicity in dress is part of faith. Simplicity in dress is part of faith." He meant plainness of appearance.
حدثنا النفيلي حدثنا محمد بن سلمة عن محمد بن إسحق عن عبد الله بن أبي أمامة
أمامة قال ذكر أصحاب رسول الله ﷺ يوما عنده الدنيا فقال رسول الله ﷺ ألا تسمعون ألا تسمعون إن البذاذة من الإيمان إن البذاذة من الإيمان يعني التقحل
[Hasan] Ibn Majah reported it in Kitab al-Zuhd, chapter "Concerning the one who is given no regard," hadith no. 4118, from the hadith of Abdullah bin Abu Umamah, through him.