Musnad Ahmad 435:88

Rawh ibn 'Ubadah narrated to us, Ibn Jurayj narrated to us, Muhammad ibn Yusuf, the freed slave of 'Amr ibn 'Uthman, informed me, from his father, from Mu'awiyah ibn Abi Sufyan, that he heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Whoever forgets something of his prayer, let him perform two prostrations while he is sitting."