A donor may not claim a deduction for any contribution of cash, a check, or other monetary gift made on or after Jan. 1, 2007, unless the donor maintains a record of the contribution in the form of either a bank record (such as a cancelled check) or a written communication from the charity (such as a receipt or a letter) showing the name of the charity, the date of the contribution, and the amount of the contribution.
Publication 1771, Charitable Contributions, Substantiation and Disclosure Requirements
Publication 526, Charitable Contributions
Notice 2006-110: Record-keeping requirements for charitable contributions made through payroll deductions.
Notice 2008-16: Rules for substantiating lump-sum charitable contributions made through the Combined Federal Campaign or a similar program (e.g., a United Way campaign).