Musannaf Ibn Abi Shayba 3:2126

Asbat ibn Muhammad narrated to us, from Mutarrif, from al-Sha'bi, who said: Al-Dahhak ibn Qays led the people in the Dhuhr prayer, and he did not sit during the first two rak'ahs. When he finished with the taslim, he performed two prostrations while he was sitting.