Musannaf Ibn Abi Shayba 3:2146

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Wahb ibn ‘Ajlan, he said: I saw al-Qasim and Salim pray behind the Imam, and he (the Imam) made a mistake (in the prayer), but he did not perform the prostration (of forgetfulness), so they did not perform the prostration.