Musnad Ahmad 435:90

Yunus narrated to us, Laith—meaning Ibn Sa'd—narrated to us, from Muhammad—meaning Ibn 'Ajlan—from Muhammad ibn Yusuf, the freed slave of 'Uthman, from his father Yusuf, from Mu'awiyah ibn Abi Sufyan: That he led them in prayer and stood up during the prayer while he should have been sitting. The people glorified Allah (said "SubhanAllah"), but he continued with his standing. Then, after he completed the prayer, he prostrated two prostrations with us while he was sitting. Then he sat upon the pulpit and said: "I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Whoever forgets something in his prayer, let him prostrate like these two prostrations.'"