Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When any of you is praying and does not know whether he has left something out of the prayer or added to it, let him perform two prostrations at the end while sitting, then give the salam."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3830