From Sahih Muslim (Hadith 1577):
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 (ﷺ)
Reference: Muslim 1577