A man questioned Ibn Umar while he was walking at Mina and said, "I vowed to fast every Tuesday or Wednesday, and this day has fallen on the Day of Sacrifice. What is your view?"Ibn Umar replied, "Allah the Exalted has commanded the fulfillment of vows, and the Messenger of Allah ﷺ forbade," or he said, "we were forbidden, to fast on the Day of Sacrifice."The man thought he had not been heard, so he said again, "I vowed to fast every Tuesday or Wednesday, and this day has fallen on the Day of Sacrifice."Ibn Umar said, "Allah has commanded the fulfillment of vows, and the Messenger of Allah ﷺ forbade us," or he said, "we were forbidden, to fast on the Day of Sacrifice."He added nothing more than that until he ascended into the mountain.
حدثنا إسماعيل أخبرنا يونس بن عبيد
سأل رجل ابن عمر وهو يمشي بمنى فقال نذرت أن أصوم كل يوم ثلاثاء أو أربعاء فوافقت هذا اليوم يوم النحر فما ترى قال أمر الله تعالى بوفاء النذر ونهى رسول الله ﷺ أو قال نهينا أن نصوم يوم النحر قال فظن الرجل أنه لم يسمع فقال إني نذرت أن أصوم كل يوم ثلاثاء أو أربعاء فوافقت هذا اليوم يوم النحر فقال أمر الله بوفاء النذر ونهانا رسول الله ﷺ أو قال نهينا أن نصوم يوم النحر قال فما زاده على ذلك حتى أسند في الجبل
Its isnad is sahih. Ismail is Ibn Ulaya. The hadith is given at greater length at 4449 and 5245. Hafiz has referred, in Fath al-Bari 4:210, to this narration in the Musnad from Ismail bin Ulaya.His words, "until he leaned against the mountain," mean he climbed up. The sanad is what rises from the ground, and it is also said to mean what faces you of a mountain and rises above its base.