It is narrated from Sayyiduna Muawiyah (may Allah be pleased with him) that he heard the Prophet (peace and blessings of Allah be upon him) say: Whoever forgets something in the prayer should sit and perform two prostrations.
Hadith Referenceمسند احمد / مسند الشاميين / 16915
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن