Sahih al-Bukhari 618

Arabic

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، قَالَ أَخْبَرَتْنِي حَفْصَةُ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا اعْتَكَفَ الْمُؤَذِّنُ لِلصُّبْحِ وَبَدَا الصُّبْحُ صَلَّى رَكْعَتَيْنِ خَفِيفَتَيْنِ قَبْلَ أَنْ تُقَامَ الصَّلاَةُ‏.‏

English translation

Narrated Hafsa:When the Mu'adh-dhin pronounced the Adhan for Fajr prayer and the dawn became evident the Prophet ordered a two rak`at light prayer (Sunna) before the Iqama of the compulsory (congregational) prayer

Reference: Sahih al-Bukhari 618, Book of Call to Prayers (Adhaan). English translation by Dr. Muhammad Muhsin Khan.