Mishnah 13 - Chapter 2 - Pirkei Avot

R. Shimon says: Be heedful of the recitation of the shema [to recite it in its proper time] and of prayer [to recite each prayer in its proper time]. And when you pray, do not make your prayer a rote thing, [thinking to yourself: When can I free myself of this duty? Or: a fixed task, as one who sets himself the task of reciting a chapter or a section (each day), in which instance he makes it mere recitation and not supplication as (does) one who implores mercy], but one's prayer should be (the imploration of) mercy, and supplication before the L rd, as it is written (Yoel 2:13): "For He is a gracious and merciful G-d, withholding wrath and abundant in lovingkindness and repenting Himself of the evil." And do not be wicked to yourself [i.e., do not do something which today or tomorrow will cause you to incriminate yourself. Rambam explains: Do not regard yourself as wicked, for that causes you to become completely corrupt. And I have heard: Do not become wicked in this matter of separating from the congregation (2:4) and being by yourself.]