It is narrated from Hazrat Abu Sa'id (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: The lower garment of a Muslim should reach to the middle of the shin; there is no harm if it is between the shin and the ankles, but the part of the lower garment that is below the ankles will be in the Hellfire.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11487
Hadith Gradingحکم دارالسلام:حديث صحيح، محمد بن إسحاق مدلس، وقد توبع