Musannaf Ibn Abi Shayba 3:2786

Hushaym narrated to us, he said: Abu Ma’shar informed us, from Muhammad ibn Qays, that the Prophet (peace and blessings of Allah be upon him), when he ordered him [the man] to pray two rak'ahs, he paused his sermon until he finished his two rak'ahs, then he returned to his sermon.