When did Prophet asked Muslims to start offering namaz?
I am sure it was in Makkah, but do not know what year.
I also read somewhere that initially only 2-rakat namaz was ordered. The other two were added later. If it is true then when were the other two added?
It’s an interesting question, I think.
Re: When did namaz become necessary?
I found out that namaz first became compulsory during miraj (621 AD). Which means only year before hijrat in 622 AD.
Still don't know about the addition of two rakats, whether it is true or not.
Re: When did namaz become necessary?
Ummat-e-Muhammad 
pr ibtada rasalt mein (Rasool Ban ny ka shuru zamana) mein Fajer or Maghrib ki 2 namaz farz thi. Hijrat (from Maccah to Madina) se dhair (1,1/2) baras pehlay jab Nabi ko Miraj how to us waqt 5 namaz farz howin.
