Muhammad, may Allah have mercy on him, in the commentary of the verse { وَالَّذِینَ لَمْ یَبْلُغُوا الْحُلُمَ مِنْکُمْ } of the Holy Qur'an, says that our family members used to teach us to greet with salam. When any of us would come, he would say "Assalamu Alaikum" and would also say, "May so-and-so enter?"