It is narrated from Umm al-Mu'minin Sayyidah Umm Salamah (may Allah be pleased with her) that he (peace and blessings be upon him) said: "The lower hem of a woman's garment should be a hand span." I said: Then her feet will show? He (peace and blessings be upon him) said: "Let them make it a forearm's length, but not exceed it."
This hadith is evidence that covering a woman's feet is obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3253