The U.S. Department of Homeland Security provides an affidavit of support, officially known as Form I-864. It is found on the U.S. Citizenship and Immigration Services website as a downloadable PDF.
An affidavit of support is a petition for a family member or other person to assume financial responsibility for an immigrant who intends to reside in the U.S. permanently. If granted, the petitioner receives an adjustment of status or an immigrant visa. The full scope of the law governing affidavits of support is found in the Immigration and Nationality Act and Title 8 of the Code of Federal Regulations.