Musannaf Ibn Abi Shayba 3:2445

Yunus ibn Muhammad narrated to us, from Abd al-Wahid ibn Ziyad, who said: Asim narrated to us, from Abu al-Aliyah, who said: He was asked about a man who is greeted with *salam* while he is in prayer. He said: When he finishes the prayer, he should follow it with the *salam*.