Q: when someone greets you by the greeting of Islam: Assalamu Alaikum?
A: Peace... It is Fardh on a muslim to reply back to the assalamu alaikum as they are saying may gods mercy and blessings be on you, and the prophet said muslims MUST say walikum salaam; as it shows the brotherhood... If someone says thank u, wuldnt u say "ur welcome?" It is fardh, NOT sunnah, its obligatory on all of us to reply back. In Chaper 4 Verse 86 "When greeted with a greeting, you shall respond with a better greeting or at least an equal one. GOD reckons all things" Remember if someone says that when u are praying, u must not reply and be concentrated on prayer, because ur mind should be only on allah, and not on anything else when in prayer.. In hadith in sahih Bukhari, Muawiya replied to a person giving salam when they were in fajr prayer, and the prophet came to him and told him, u shuld be fully concentrated on God when u are praying and should not reply to any1. Peace