Abu Sa'id Al-Khudri (May Allah be pleased with him) reported:
Messenger of Allah (ﷺ) said, "The lower garment of a believer should be half way below the knees. He is guilty of no sin if they are up to the ankles. That which is below the ankles is in the Fire (of Hell). Allah will not look at one who allows his lower garment to trail out of vanity.[Abu Dawud].
App reference: Book 4, Hadith 21
Arabic/English book reference: Book 4, Hadith 799