Musnad Ahmad 482:16

Waki’ narrated to us, he said: Sufyan narrated to us, from Yahya ibn Sa’id, from ‘Ubayd Allah ibn Zahr, from Abu Sa’id al-Ru’ayni, from ‘Abd Allah ibn Malik al-Yahsubi, from ‘Uqbah ibn ‘Amir al-Juhani, that his sister vowed to walk barefoot and unveiled. So he asked the Prophet, peace and blessings of Allah be upon him, and he said: "Indeed, Allah has no need for the misery of your sister. Command her to cover her head, to ride, and to fast for three days."