Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1567
'A'isha reported: The Messenger of Allah (may peace be upon him) used to observe two rak'ahs of Sunnah (prayer) when he heard the Adhin and shortened them. (This hadith has been narrated by the same chain of transmitters and in the hadith narrated by Usama the words are:" When it was dawn".)