Sahih Muslim Book 4, Hadith Number 1567.

Chapter : Excellence of the forenoon prayer, two are its minimum Rak’ahs and eight are its maximum Rak’ahs and average Rak’ahs are four or six, and exhortation for the observance of the prayer.

‘Aisha 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”.)