Aeronautical Information Manual - AIM
Chapter 6. Emergency Procedures
Section 3. Distress and Urgency Procedures
https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap6_section_3.html
"6-3-1. Distress and Urgency Communications" ...
"C. The initial communication, and if considered necessary, any subsequent transmissions by an aircraft in distress should begin with the signal MAYDAY, preferably repeated three times. The signal PAN-PAN should be used in the same manner for an urgency condition."