(a) Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) led the Zuhr prayer with five rak'ahs. He (peace and blessings be upon him) was asked: Has something been added to the prayer? He (peace and blessings be upon him) said: "How so, or what addition is that?" It was said: You prayed five rak'ahs. So he (peace and blessings be upon him) performed two prostrations while sitting; or he said: after he had finished.
(b) Imam al-Bukhari (may Allah have mercy on him) narrated this hadith in his Sahih from Abu al-Walid (may Allah have mercy on him), that he (peace and blessings be upon him) performed two prostrations after giving the salam; and this is because they mentioned it to him only after the salam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3838