Per 2017 Instructions for Schedule A (Form 1040) Line 17 Page A-9:
Gifts of $250 or more. You can deduct a gift of $250 or more only if you have a statement from the charitable organization showing the information in (1) and (2) next.
In figuring whether a gift is $250 or more, don't combine separate donations. For example, if you gave your church $25 each week for a total of $1,300, treat each $25 payment as a separate gift. If you made donations through payroll deductions, treat each deduction from each paycheck as a separate gift. See Pub. 526 if you made a separate gift of $250 or more through payroll deduction.
You must get the statement by the date you file your return or the due date (including extensions) for filing your return, whichever is earlier. Don't attach the statement to your return. Instead, keep it for your records.