Musannaf Ibn Abi Shaybah

Hazrat Zaid bin Aslam narrates that my father sent me to Hazrat Ibn Umar (may Allah be pleased with them). So I said to him: "Shall I come in?" He replied: "Do not say it like this, rather say: 'Assalamu Alaikum.' When it is said to you, 'Wa Alaikum Assalam,' then you may enter."