Musnad Ahmad 626:90

Hajjaj narrated to us, he said: I heard Sufyan, 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) said: "If one of you stands up and has not fully straightened up while standing, let him sit down. And if he has fully straightened up while standing, let him not sit down, and let him perform the two prostrations of forgetfulness."