Receipt Notice (I-797)
The Form I-797 USCIS mails after you file, with your receipt number and filing date.
The receipt notice, Form I-797, is the first official document USCIS sends after you file. It confirms your case was received and gives you a receipt number, your filing date, and the office handling your case.
That receipt number is what you use to check case status online and to look up your processing time. Keep it somewhere safe, because almost every later step asks for it.
For some forms like I-751, the receipt notice also extends your status while you wait, so it can be as important as the green card itself.