Umar had made a vow, meaning to perform spiritual retreat in the Sacred Mosque, so he asked the Prophet ﷺ about it, and the Prophet instructed him to fulfill it.Sufyan was asked, "Did Ayub narrate from Nafi from Ibn Umar that Umar made a vow?"He replied, "Yes."
سمعت
إنه نذر يعني أن يعتكف في المسجد الحرام فسأل النبي ﷺ فأمره قيل لسفيان عن أيوب عن نافع عن ابن عمر أن عمر نذر قال نعم
Its isnad is sahih, and it is an abridgment of 255, but there it is narrated from Ibn Umar from Umar, making it part of the Musnad of Umar. Sufyan abridged the wording of the hadith here, but the meaning is clear, that the Prophet ﷺ commanded him to fulfill his vow.