We went out with Ibn Umar to the protected pasture. When the sun set, we hesitated to remind him about the prayer until the whiteness of the horizon had faded and the early darkness of night had set in. Then he dismounted and led us in three units and two units of prayer. He turned to us and said, "This is how I saw the Messenger of Allah ﷺ do it."
حدثنا سفيان عن ابن أبي نجيح عن إسماعيل بن
خرجنا مع ابن عمر إلى الحمى فلما غربت الشمس هبنا أن نقول له الصلاة حتى ذهب بياض الأفق وذهبت فحمة العشاء نزل فصلى بنا ثلاثا واثنتين والتفت إلينا وقال هكذا رأيت رسول الله ﷺ فعل
Its isnad is sahih. The reserved pasture referred to is apparently Naqi (with an n), a place near Madinah, twenty leagues from it. The Prophet ﷺ had reserved it for his horses, and Umar bin Khattab later reserved it for the horses of the Muslims. See 4542.