(a) It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) prayed Zuhr as five rak'ahs. When he gave the salam ending the prayer, someone said: "Has something been added to the prayer?" He (peace and blessings be upon him) said: "No." The Companions said: "But you prayed five rak'ahs," so he (peace and blessings be upon him) performed two prostrations.
(b) Another narration does not mention Zuhr; in it, it says that when he turned away from the prayer, he performed the two prostrations of forgetfulness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3880