Musnad Ahmad 27:325

Waki’ narrated to us, al-‘Umari narrated to us, from Nafi’, from Ibn ‘Umar: "That the Messenger of Allah (ﷺ) granted a concession to the women to let [their garments] hang down by a span." They said: "O Messenger of Allah, then our feet will be uncovered." He said: "A forearm’s length, and do not exceed that."