I entered the second stage of a job interview and after the second stage I received a mail that the feedback from my interview was positive, therefore I should send some documents while the sourcing process continues.

I sent the required documents and soon again I got another mail that I should provide the contact details of one of my past employer since they will be conducting a background reference check on me, I immediately provided all the required information.

However, I never heard from them for about 10days until now that I got a mail informing me I have been kept in view but for now I would not be proceeding in the sourcing process.

I want to respond to the mail, but what do I say?

I would suggest something like this:

"Dear so-and-so, thank you for the consideration. I enjoyed the opportunity to meet and discuss my experience and qualifications. Please keep me in mind if other positions open up. I would also appreciate if there is any specific feedback you can share with me on my interview or on my qualifications I would sincerely appreciate it. Thank you."

It would be great if they could give you that feedback, but be aware that in many companies hiring managers are specifically warned to NOT give feedback to job candidates that do not get an offer, but let HR manage any and all interactions once the hiring decision is made. That's the case with my employer, and in fact we are instructed to shred all hand-written notes once the search is completed. So don't be surprised (or too disappointed) if you get no response.

This is really helpful, I am indeed grateful. Thanks

All you know is that you didn't get the job. Nothing to indicate 'unfavorable review.'
For all you know, the CEO's nephew suddenly needed a job.
I know it's a letdown, but take a deep breath and say OH WELL.