The Rules of Golf do not address the issue of the validity of a hole in one. The USGA recommends that a hole in one be considered valid:

• If made during a round of at least nine holes, except that a hole in one made during a match should be acceptable even if the match ends before the stipulated round is completed
• If the player is playing one ball; a hole in one made in a practice round in which the player is playing two or more balls should not be acceptable
• If attested by an acceptable witness
• If made at a hole with a temporary tee and/or putting green in use, even if the Committee did not specifically define the teeing ground with tee-markers; the length of the hole at the time should be stated on any certificate
• If made in a “scramble” competition, for example, a Captain’s Choice