Musnad Ahmad 626:89

Aswad ibn Amir narrated to us, Isra’il narrated to us, from Jabir, from al-Mughirah ibn Shibl, from Qays ibn Abi Hazim, from al-Mughirah ibn Shu’bah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in the Dhuhr or the Asr prayer, and he stood up [when he should have sat]. We said, "Subhan Allah." He said, "Subhan Allah," and gestured with his hand, meaning: "Stand up." So we stood up. When he finished his prayer, he performed two prostrations [of forgetfulness]. Then he said: "If one of you remembers before he has fully stood up, let him sit down. And if he has fully stood up, let him not sit down."