Money transfer to mother from son out of savings abroad

I have a situation regarding income tax wrt money transferred in my mother's account by me.I went to USA & used to save money out of (the only source of income) my salary and I used to transfer the money in India to my mother's account.I did 9 such transactions in a span of last 3 years & the amount transferred in my mother's saving account varied from Rs 2,000 to Rs 4,50,000. So money in Dollars from my US account was transferred directly to my mother's account in India. I obviously have full proof for source of this income/amount (US Payslips & US income tax forms W2 like Form 16 in India) which was saved as part of earning through SALARY only in USA (after paying tax over there).

This resulted in accumulation of money in my mother's saving account.At that time I was not aware of the concept of gift deed and tax implications on FDs.I did lots of FDs in my mother's account on which obviously tds was deducted by bank. My mother has no income of any kind as she is a house wife. She has a PAN card though.I filed her ITR in order to get refund.
 

Now in case of any queries raised by income tax officers regarding the source of amount , I think it will be perfectly legal to say that her son has GIFTED the amount (in CASH) by internet banking transfer supported by all bank statements (from son & mother).

I wish to document the cash transfer done in last 3 years to settle all audit matters to rest.
Can I prepare it on a stamp paper of Rs 100 (purchase it now) & mention all 9 transactions done over last 3 years (supported by bank statements) in it & notarize it.
Will there be any objections by income tax officers for doing the documentation after 3 years?
Will the gift deed / documentation date should be a date before the date of filing tax returns for my mother?

I fear that I may be asked why the documentation is done now at the time of filing her income tax returns. I just wish to be legally safe.I have done lot of research on net & read many blogs/forum on many sites including this one & then when I could not figure out how to document the back dated gift, I opted for writing in forum in order to get expert advise. Can someone help with my queries? Your help would be greatly appreciated!!!