Mozart on rubato in adagios

In 1777, Mozart visited Heir Stein in Ausburg (1). According to Mozart, Stein had stated that no-one

has ever played his Piano Forte as well as I have, and, besides, I always keep correct time. They are all wondering about that. They simply can’t believe that you can play a Tempo rubato in an Adagio, and the left hand knows nothing about it but goes on playing in strict time. As far as they know, the left hand always follows the right. (2)

(1) Johann Andreas Stein built the first piano with escapement action, allowing rapidity of movement.

(2) Mozart, in a letter to his father, 23-25 October 1777. Cited in: Spaethling, Robert (2000) Mozart’s Letters; Mozart’s Life. London: Faber and Faber, p.81.