Sayyiduna Ibn Buhaina (may Allah be pleased with him) narrates: The Prophet Muhammad (peace and blessings be upon him) performed prayer. After completing two rak‘ahs, he stood up. When he was supposed to sit, we said "Subhan Allah," but he continued the prayer. When he finished the prayer and was sitting, he performed two prostrations of forgetfulness (sajda sahw).
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2680
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «الصحيحة» (2457): ق.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده على شرط البخاري