Mishnah 1 - Chapter 2 - Pirkei Avot

Rabbi omer. Eizohi derech yesharah sheyavor lo ha'adam, kol shehi tif'eret le'oseiha vetif'eret lo min ha'adam. Vehevei zahir bemitzvah kallah kevachamurah, she'ein attah yodea mattan secharan shel mitzvot. Vehevei mechashev hefsed mitzvah keneged secharah, vschar averah keneged hefsedah. Vehistakkel bishloshah devarim ve'i attah va lidei averah - da mah lema'lah mimmecha, ayin ro'ah ve'ozen shoma'at, vechol ma'aseicha bassefer nichtavin.

Rabbi Said: which is the straight path that a man should choose for himself? One which is an honor to the person adopting it, and [on account of which] honor [accrues] to him from others. And be careful with a light commandment as with a grave one, for you did know not the reward for the fulfillment of the commandments. Also, reckon the loss [that may be sustained through the fulfillment] of a commandment against the reward [accruing] thereby, and the gain [that may be obtained through the committing] of a transgression against the loss [entailed] thereby. Apply your mind to three things and you will not come into the clutches of sin: Know what there is above you: an eye that sees, an ear that hears, and all your deeds are written in a book.

Rabbi says: Which is the just path that a man should choose for himself? All that is gratifying to its doer and that earns him the praise of man. [And this will obtain when he walks in all the middoth (character traits) in the middle path and does not incline to one of the two extremes. For if he is extremely stingy, this is gratifying to him, for he thereby amasses great wealth, but men do not praise him for this. And if he is vainly extravagant, the men who receive from him praise him, but this (extravagance) is not gratifying to its doer, for he thereby reduces himself to poverty. But the middah of generosity, which is the mean between stinginess and extravagance, is gratifying to its doer, for he thereby preserves his wealth and is not overly extravagant, and this earns him the praise of men for living as he should. The same is true of all of the middoth]. And take heed of a "light" mitzvah as of a "heavy" one, for you do not know the reward of mitzvoth. [The Torah does not specify the reward of one who fulfills a positive commandment nor the punishment of one who fails to fulfill it. For the punishments of the negative commandments are specific: stoning, burning, the sword, strangulation, cutting-off, death at the hands of Heaven, stripes — the light punishment for the "light" transgression, and the heavy punishment for the "heavy" one], and weigh the "loss" of (performing) a mitzvah [i.e., what you lose of your wares or your money by engaging in a mitzvah] against its reward [in this world or the next, which will exceed that loss], and the "reward" of a transgression [i.e., what you gain from it] against its loss [i.e., what you are destined to lose by it.], and contemplate these things and you will not come to transgression: Know what is above you — a seeing eye and a hearing ear and all of your deeds are recorded in a book.