Musannaf Ibn Abi Shayba 3:2123

Narrated to us Abd al-A’la, from Yunus, from al-Hasan, regarding a man who prayed two rak'ahs of an obligatory prayer and forgot to perform the tashahhud until he stood up. He said: "When he is established in a standing position, he should continue in his prayer and perform the two prostrations of forgetfulness."