Irs want supporting document for my entry of $1393 for federal income tax withheld. This amount is listed on line 40 on 1040a form.. what exactly do I have to attach?

Attach the documents that show the withholding, such as your W2 or 1099.

I do not have w2 or 1099.. it was seduction for mileage and gas which I spent to travel to work.  How can I have w2 for that??

I just followed turbo tax step by step and that figure was made by turbo tax.. I didn’t randomly chose that amount.

In this case, send a copy of your mileage log, but that is not what is reported on line 40 on a 1040A. Line 40 on a 1040A reports withholding. It is reporting that you paid 1393 as a tax. Send the IRS what they ask for and what you have, but if you claimed tax paid, and there wasn't any, you may need to pay the IRS. If this is the case and this is for 2017, you can pay the additional tax before April 17th to avoid interest. I do not know how mileage could be reported as tax in the TurboTax program. If this was for travel to work, why don't you have a W2 or 1099 or Self-Employment Schedule C?