Musannaf Ibn Abi Shayba 3:2122

Yahya ibn Abd al-Malik ibn Abi Ghaniyah narrated to us, from Abd al-Malik, from Ata, regarding a man who prays two rak'ahs of an obligatory prayer then stands up: He said: If he remains standing, he continues in his prayer. Then, when he completes his prayer, he prostrates two prostrations while he is sitting after he has performed the taslim.