Musnad Ahmad 482:84

Yahya ibn Sa'id al-Qattan narrated to us, from Yahya ibn Sa'id, and Yazid ibn Harun, who said: Yahya ibn Sa'id informed us, Ubayd Allah ibn Zahr narrated to me, that Abu Sa'id—Yazid said: al-Ru'ayni—informed him, that 'Abd Allah ibn Malik informed him, that 'Uqbah ibn 'Amir informed him, that he asked the Prophet, peace and blessings of Allah be upon him, about a sister of his who had vowed to walk barefoot and unveiled. The Prophet, peace and blessings of Allah be upon him, said: "Let her cover her head, let her ride, and let her fast for three days."