(14) باب اسْتِحْبَابِ رَكْعَتَيْ سُنَّةِ الْفَجْرِ وَالْحَثِّ عَلَيْهِمَا وَتَخْفِيفِهِمَا وَالْمُحَافَظَةِ عَلَيْهِمَا وَبَيَانِ مَا يُسْتَحَبُّ أَنْ يُقْرَأَ فِيهِمَا
'A'isha reported:
The Messenger of Allah (ﷺ) used to observe two rak'ahs of Sunnah (prayer) when he heard the Adhin and shortened them.
Reference: Sahih Muslim 724 a
In-book reference: Book 6, Hadith 111
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1567