A declaration is a simple statement that you intend in good faith to return to the United States to resume your domicile. If you want to sponsor someone for a family-based green card, you need to provide proof of a U.S. domicile in your Affidavit of Support (Form I-864).Typically, in order to claim US domicile, the Affidavit of Support sponsor must be residing in the United States. How Can I Prove U.S. Domicile? If you live outside the US but claim to be a US citizen, you will need to send in a written explanation and proof of your claim along with your Form I864. To prove that you live in California, you must present two documents that contain your California physical address.