Sahih Muslim Book 4, Hadith Number 1565.

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.

A Hadith like this has been narrated by Shu’ba with the same chain of transmitters.