Musar for Pirkei Avot 2:13
רַבִּי שִׁמְעוֹן אוֹמֵר, הֱוֵי זָהִיר בִּקְרִיאַת שְׁמַע וּבַתְּפִלָּה. וּכְשֶׁאַתָּה מִתְפַּלֵּל, אַל תַּעַשׂ תְּפִלָּתְךָ קֶבַע, אֶלָּא רַחֲמִים וְתַחֲנוּנִים לִפְנֵי הַמָּקוֹם בָּרוּךְ הוּא, שֶׁנֶּאֱמַר (יואל ב) כִּי חַנּוּן וְרַחוּם הוּא אֶרֶךְ אַפַּיִם וְרַב חֶסֶד וְנִחָם עַל הָרָעָה. וְאַל תְּהִי רָשָׁע בִּפְנֵי עַצְמְךָ:
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.]