Currently I have F-1 visa status and I applied for TPS on 6 July 2015 almost 120 days and I have not received anything yet. I just done my biometrics on August 16 2015 and even my EAD is not approved yet.
If I have to start working do i need a valid EAD.
My question is, if the time comes and still I have not heard back from USCIS, will it be OK to work while my status is pending providing my receipts number and alien number ?

Any work before getting an EAD is an unauthorized work.

You can call and ask for a service request as its been too long since you filled your application.

Good luck.

TSB_Adhish: I totally agree with mero_naam_ho. I had applied in August and had biometrics in September however, my case is pending too. I have no idea why they are delaying.
I would also insist you DO NO WORK IF YOU DONT HAVE EAD, its a risk. Wait for few weeks for your case to get approved and then start work to be on safe side.