Musannaf Ibn Abi Shayba 3:2076

Yazid ibn Harun narrated to us, from Ash'ath, from al-Sha'bi, from Sa'd and Ammar, that they prayed three [rak'ahs] then performed the taslim. It was said to them [about that], so they performed the one that remained for them, then they said "Allahu Akbar," then they prostrated [the prostrations of forgetfulness], then they performed two taslims.