Musannaf Ibn Abi Shayba 3:4025

Abu Bakr narrated to us, he said: Mu'tamir ibn Sulayman narrated to us, from Layth, from Mujahid, who said: Ibn Mas'ud saw a man speaking to another after the two rak'ahs of the dawn (prayer). He said: "Either you both remember Allah, or you both remain silent."