Ibn ‘Umar reported that the Prophet (saw) said:
“Whoever trails his garment out of pride, Allaah will not look at him on the day of judgement.“ So Umm Salamah (ra) asked: “Then what should the women do with the hems of their dresses?“ The Prophet (saw) said: “Let them extend their hems the length of a hand span.“ She said: “But their feet would still be exposed.“ So he replied: “Then let them extend it a forearm’s length and no more.” [Aboo Daawood]
Ibn ‘Uthaymeen comments:
“There is evidence in this hadeeth that a woman is obligated to cover her feet. This was a well known matter amongst the female companions. Without a doubt, the foot is less a place of fitnah than the face and hands.
Thus, warning against something that is a lesser danger also consists of a warning against what is greater and superior than it, based on the wisdom of the Last Revelation.
Would it obligate the covering of an area that is less a place of fitnah, and allow the exposure of that which is a greater area of fitnah? Indeed this is a clear contradiction that is not possible for the wisdom of Allaah and His Legislation.”
[Taken From “The Four Essays On The Obligation Of Veiling”, Ch. 2: “An Essay On Hijaab” By Ibn ‘Uthaymeen, p.39]