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 ‏.‏

It was narrated from Hafsah that: When the call for Subh prayer was given, the Messenger of Allah (PBUH) would pray two brief rak'ahs before going to the prayer. Translation Not Available

Book Ref: Sunan An Nisai Book 20 Hadith 1760
Web Ref:  Sunan An Nisai Vol 2 Book 20 Hadith 1761