Yeshayahu Leibowitz Quote

Only the prayer which one prays as the observance of a Mitzvah is religiously significant. The spontaneous prayer ("when he is overwhelmed and pours out his complaint before God") a man prays of his own accord is, of course, halakhically permissible, but, like the performance of any act which has not been prescribed, its religious value is limited. As a religious act it is even faulty, since he who prays to satisfy his needs sets himself up as an end, as though God were a means for promotion of his welfare.

"Judaism, Human Values and the Human State" (1995)