It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that she mentioned the hems of women's garments to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) said: "Let them hang down a hand-span." I said: Then their (feet) will be exposed. He (peace and blessings be upon him) said: "Then a forearm's length; they should not add to it."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19883