When dawn began to brighten, the Messenger of Allah ﷺ would pray two rakahs.
حدثنا سفيان
أن رسول الله ﷺ كان إذا أضاء الفجر صلى ركعتين
No references are available.
Its isnad is sahih, and it is an abridgment of 4506, see also 4660.