Whenever the muezzin gave the call to the dawn prayer and dawn began to break, the Messenger of Allah ﷺ would pray two brief units of prayer before the iqamah was called.
أن رسول الله ﷺ كان إذا اعتكف المؤذن للصبح وبدأ الصبح صلى ركعتين خفيفتين قبل أن تقام الصلاة.