Ahmad ibn Muhammad from Uthman ibn Isa from Abu Abdillah عليه السلام, peace be upon him, who said: I asked Imam عليه السلام about a man who is greeted with salam while he is in prayer. Imam عليه السلام said: "He should respond by saying, 'Salamun alaykum,' and he should not say, 'Alaykumu as-salam,' for the Messenger of Allah سبحانه وتعالى, may Allah سبحانه وتعالى bless him and his family, was once standing in prayer when Ammar ibn Yasir passed by Him صلى الله عليه وعلى آله وسلم and greeted Him صلى الله عليه وعلى آله وسلم with salam, so the Prophet صلى الله عليه وعلى آله وسلم, may Allah سبحانه وتعالى bless him and his family, responded to him in this manner."