A man from the Ansar was admonishing his brother for being too modest, so the Prophet ﷺ said, "Leave him, for modesty is part of faith."
حدثنا يحيى بن سعيد حدثنا مالك
أن رجلا من الأنصار كان يعظ أخاه في الحياء فقال النبي ﷺ دعه فإن الحياء من الإيمان
Its chain of transmission is sahih, and it is a repetition of 4554. Ibn Athir said, "He made modesty, which is an instinct, part of faith, which is something acquired, because the modest person is restrained by his modesty from sins, even if he has no fear of consequences, so it becomes like faith, which restrains a person from sins. He only made it a part of faith because faith is divided into obeying what Allah has commanded and refraining from what Allah has forbidden. So when refraining is achieved through modesty, it constitutes part of faith."