Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1761 (DarusSalam) ⓘ
It was narrated from Hafsah that: When the call for Subh prayer was given, the Messenger of Allah would pray two brief rak'ahs before going to the prayer.
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنْ حَفْصَةَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ كَانَ إِذَا نُودِيَ لِصَلاَةِ الصُّبْحِ رَكَعَ رَكْعَتَيْنِ خَفِيفَتَيْنِ قَبْلَ أَنْ يَقُومَ إِلَى الصَّلاَةِ .
also referenced under: Hadith 1760 (Reference) & Book 20, Hadith 163 (In-Book)