Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1778 (DarusSalam) ⓘ
It was narrated from Ibn Umar, from Hafsah, that: When the call to Subh prayer was given, the Messenger of Allah would pray two brief rak'ahs before going to pray.
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 . وَرَوَى سَالِمٌ عَنِ ابْنِ عُمَرَ عَنْ حَفْصَةَ .
also referenced under: Hadith 1777 (Reference) & Book 20, Hadith 180 (In-Book)