It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him), who attributes it to the Prophet (peace and blessings be upon him), that he said: "When a man becomes confused in his prayer and does not know whether he has added or omitted something, let him perform two prostrations while sitting."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5421