Musannaf Ibn Abi Shayba 17:373

Yahya ibn Muhammad al-Qurashi Abu Zakir narrated to us, saying: I heard Zayd ibn Aslam say: My father sent me to Ibn Umar. I said, "May I enter?" He said, "Do not say it like that, but say, 'Peace be upon you' (Assalamu Alaikum). Then, if it is said to you, 'And upon you be peace' (Wa Alaikum), then enter."