Sunan an-Nasa'i 3822

Ahmad ibn ʿAbdullah ibn al-Hakam reported to us; he said: Muhammad ibn Jaʿfar narrated to us; he said: Shuʿbah narrated to us; he said: I heard ʿUbaydullah narrate from Nafiʿ, from Ibn ʿUmar, that ʿUmar had committed himself to spend one day in devotional retreat during the pre-Islamic era. He asked the Messenger of Allah, peace and blessings be upon him, about it, and he instructed him to observe that retreat.