As oicur says it’s in the FAA AIM -
5–3–3 Additional Reports
a. The following reports should be made to ATC or FSS facilities without a specific ATC request:
1. At all times.
(a) When vacating any previously assigned altitude or flight level for a newly assigned altitude or flight level.
(b) When an altitude change will be made if operating on a clearance specifying VFR-on-top.
(c) When unable to climb/descend at a rate of a least 500 feet per minute.
(d) When approach has been missed. (Request clearance for specific action; i.e., to alternative airport, another approach, etc.)
(e) Change in the average true airspeed (at cruising altitude) when it varies by 5 percent or 10 knots (whichever is greater) from that filed in the flight plan.
(f) The time and altitude or flight level upon reaching a holding fix or point to which cleared.
(g) When leaving any assigned holding fix or point.