Sahih Muslim Book - 6, Hadith - 1676

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ أَنَّ حَفْصَةَ أُمَّ الْمُؤْمِنِينَ أَخْبَرَتْهُ أَنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- كَانَ إِذَا سَكَتَ الْمُؤَذِّنُ مِنَ الأَذَانِ لِصَلاَةِ الصُّبْحِ وَبَدَا الصُّبْحُ رَكَعَ رَكْعَتَيْنِ خَفِيفَتَيْنِ قَبْلَ أَنْ تُقَامَ الصَّلاَةُ.

It was narrated from Ibn 'Umar that Hafsah, the Mother of the Believers, told him that when the Mu'adhdhin fell silent following the Adhan for the Subh prayer, and dawn had begun, the Messenger of Allah (s.a.w) would pray two brief Rak'ah before the Iqamah for prayer was called."