Hadithcore
Sahih Muslim, 724 a
sahih

'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.

حَدَّثَنَا عَمْرٌو النَّاقِدُ، حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، حَدَّثَنَا هِشَامُ بْنُ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي رَكْعَتَىِ الْفَجْرِ إِذَا سَمِعَ الأَذَانَ وَيُخَفِّفُهُمَا ‏.‏

References2 variants
In-Book Reference
Book 6, Hadith 111
USC-MSA web (English) reference
Book 4, Hadith 1567 (deprecated numbering scheme)
Sharh · explanationclick to expand
Islam encouraged and urged the performance of supererogatory prayers and made them greatly rewardable. One of these supererogatory prayers, whose performance was urged by the Prophet (may Allah's peace and blessings be upon him), is the two-Rak‘ah Sunnah of the Fajr prayer. In this Hadīth, the Mother of the Believers ‘Ā’ishah (may Allah be pleased with her) says that the Prophet (may Allah's peace and blessings be upon him) was not more persistent in observing any supererogatory prayer than he was in performing two Rak‘ahs before Fajr. In other words, he was not persistent and keen to perform any of the supererogatory prayers and regular Sunnah prayers as he was in offering two Rak‘ahs before the two Rak‘ahs of Fajr, which is the Sunnah before the Fajr prayer. They are counted among the regular supererogatory prayers associated with the obligatory prayers. The Prophet's keenness on them indicates the significance of their status and reward..
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