Talmud 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.]
Avot D'Rabbi Natan
He said to them: Go out and see what is the evil path that a person should stay away from, so that he can enter into the World to Come. Rabbi Eliezer came back and said: An evil eye. Rabbi Yehoshua came back and said: An evil friend. Rabbi Yosei came back and said: An evil eye, an evil neighbor, and an evil wife. Rabbi Shimon came back said: One who borrows money and does not pay it back. For one who borrows from people will be punished by God, as it says (Psalms 37:21), “The wicked one borrows and does not repay; the righteous one is generous and keeps giving.” Rabbi Elazar came back and said: An evil heart toward Heaven, an evil heart toward the mitzvot, and an evil heart toward others. [Rabban Yohanan ben Zakkai] said to them: I prefer Rabbi Elazar ben Arach’s words, because I see all of your words contained within his words.