14 CFR § 61.189

A flight instructor must sign the logbook of each person to whom that instructor has given flight training or ground training.

A flight instructor must maintain a record in a logbook or a separate document that contains the following:

The name of each person whose logbook that instructor has endorsed for solo flight privileges, and the date of the endorsement; and

The name of each person that instructor has endorsed for a knowledge test or practical test, and the record shall also indicate the kind of test, the date, and the results.

Each flight instructor must retain the records required by this section for at least 3 years.