Friederike Müller, a pupil of Chopin, wrote: "[His] playing was always noble and beautiful; his tones sang, whether in full forte or softest piano. He took infinite pains to teach his pupils this legato, cantabile style of playing. His most severe criticism was 'He—or she—does not know how to join two notes together.' He also demanded the strictest adherence to rhythm. He hated all lingering and dragging, misplaced rubatos, as well as exaggerated ritardandos ... and it is precisely in this respect that people make such terrible errors in playing his works."
According to who did Chopin demand strictly sticking with rhythm?
Friederike Müller