Hi Shobhit,
I just saw your comment, and IMHO it cannot be considered a bug. In fact it has always been possible to save business transactions with errors in SAP CRM (either because information is missing, or inconsistent, or whatever the reason). So until recently (let's says a couple of years) you had to implement badi ORDER_SAVE to prevent a business transaction from being saved depending on your own rules.
However, as many customers complained about it, SAP delivered the following OSS note which implements a new parameter in table SMOFPARSFA like Hasan said above (see also all related notes):
Note 1353553 - Opportunity: Behavior of mandatory fields in Web UI
So it is up to the customer to choose whether or not a business transaction can be saved if mandatory fields are empty on WebUI. Note that it will have no influence on SAP GUI (so you could still create business transaction with mandatory fields left empty), or BAPI, IDoc processing, etc. The note is valid for WebUI only.
By the way, I find it surprising to see that this question is still marked as open: I think the correct answer has been provided many times (and only a few hours after it was created). So maybe Ankita Chhabra can close it now to make the solution more easy to find on SCN search engine?
Thanks,
Nicolas.