Riyad as-Salihin #117

Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said to a man who was performing Salah while his lower garment was trailing, "Go and perform your Wudu' again." The man went and came back having performed it. The Messenger of Allah (ﷺ) said (again), "Go and perform your Wudu'." Someone present said to the Messenger of Allah (ﷺ): "O Messenger of Allah! You ask him to perform his Wudu' and then you kept silent (without saying the reason for it)." He (ﷺ) said, "He performed Salah while his lower garment was below his ankles. Allah does not accept the Salah of a man who trails his lower garment.".