He asked the Prophet ﷺ about a sister of his who had vowed to walk barefoot and unveiled. The Prophet ﷺ said to him, "Order her to cover her head, to ride, and to fast three days."
أخبرنا عمرو بن علي ومحمد بن المثنى قالا حدثنا يحيى بن سعيد عن يحيى بن سعيد عن عبيد الله بن زحر وقال عمرو إن عبيد الله بن زحر أخبره عن عبد الله بن مالك أن
أنه سأل النبي ﷺ عن أخت له نذرت أن تمشي حافية غير مختمرة فقال له النبي ﷺ مرها فلتختمر ولتركب ولتصم ثلاثة أيام
[Its chain is daif] Abu Dawud narrated it in "Oaths," in the chapter "On the one who considers atonement due when it involves disobedience," no. 3293, from the hadith of Yahya bin Saeed Qattan, from Yahya bin Saeed Ansari, with this wording. It is also found in "Al-Kubra," no. 4757. Tirmidhi said in no. 1544 that it is hasan.Ubaydullah bin Zahr is daif, and the majority of scholars have graded him as daif. There is a daif corroborating narration for it in Ahmad, 4/147.