“Number theory is useful, since one can graduate with it.”
Die Zahlentheorie ist nützlich, weil man mit ihr promovieren kann.
Foreword to Vorlesungen über Zahlentheorie (Lectures on Number Theory) (1927).
Edmund Georg Hermann Landau was a German mathematician who worked in the fields of number theory and complex analysis. Wikipedia

“Number theory is useful, since one can graduate with it.”
Die Zahlentheorie ist nützlich, weil man mit ihr promovieren kann.
Foreword to Vorlesungen über Zahlentheorie (Lectures on Number Theory) (1927).
Grundlagen der Analysis [Foundations of Analysis] (1930) Preface for the Student, as quoted by Eli Maor, Trigonometric Delights (2013)
“Please forget everything that you have learned in school; for you haven't learned it.”
Foundations of Analysis (1960) as quoted by Eli Maor, The Pythagorean Theorem: A 4,000-year History (2007)
Grundlagen der Analysis [Foundations of Analysis] (1930) Preface for the Teacher, as quoted by Eli Maor, Trigonometric Delights (2013)
“We mathematicians are all a little bit crazy.”
Wir Mathematiker sind alle ein bißchen meschugge. <br class="br">quoted by Paul Erdős https://www.youtube.com/watch?v=nCQwh9fsU7I&t=20m24s
“Please don't read the preface for the teacher.”
Grundlagen der Analysis [Foundations of Analysis] (1930) Preface for the Student, as quoted by Eli Maor, Trigonometric Delights (2013)
Grundlagen der Analysis [Foundations of Analysis] (1930) Preface for the Student, as quoted by Eli Maor, Trigonometric Delights (2013)