Another narration states that the Prophet (peace and blessings be upon him) prayed the Zuhr prayer with five rak'ahs. When he (peace and blessings be upon him) gave the salam, someone said: Has something been added to the prayer? He (peace and blessings be upon him) said: "How so?" The Companions (may Allah be pleased with them) said: You have prayed five rak'ahs. So he (peace and blessings be upon him) performed two prostrations.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3839