Musnad Ahmad 482:57

Ibn Numayr narrated to us, Yahya bin Sa’id narrated to us, from ‘Ubayd Allah bin Zahr al-Damri, that he heard Abu Sa’id al-Ru’ayni narrating that ‘Abd Allah bin Malik informed him, From ‘Uqbah bin ‘Amir al-Juhani, who informed him: That his sister made a vow to walk barefoot and unveiled. ‘Uqbah mentioned that to the Messenger of Allah, peace and blessings of Allah be upon him. And the Messenger of Allah, peace and blessings of Allah be upon him, said: "Command your sister to ride and to cover her head, and to fast for three days."