I haven't tracked down the original rule, but according to a law firm's website at
rules ... require every fax (not just unsolicited advertisements) to identify in the top or bottom margin on each page or on the first page the date and time it is sent, the sender's identity (the originator of the fax, not the name of the broadcast fax service), and the fax number of the machine sending the transmission or the telephone number of the sender. If a broadcast fax service (which likely includes an outside marketing firm retained for such purposes) is used and the fax broadcaster is responsible for the content of the fax or for supplying the fax numbers to which the faxes are sent, then the fax broadcaster's name also must be identified in the header.