Sahih Muslim Book 4, Hadith Number 1465.

Chapter : Compensation of the missed prayer and excellence of observing it promptly.

Musa b. Salama Hudhali said: I asked Ibn ‘Abbas: How should I say prayer when I am in Mecca, and when I do not pray along with the Imam? He said: Two rak’ahs (of prayer) is the Sunnah of Abu’l-Qasim (may peace be upon him).