Avos 2:1

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.

Also, weigh the loss [that may be sustained through the fulfillment] of a commandment against the reward [that may be obtained] for [fulfilling] it: As if your heart will whisper something to say, "How will I do this and lose my money with this and that"; you can also say to it, "Because there will be a great reward for it, two thousand times the loss." And with this, it will not prevent you from glory.

And [weigh] the gain [that may be obtained through the committing] of a transgression against the loss [that may be sustained] by [committing] it: 'Lest there be in your heart the base thought to say,' "There is great reward in doing this sin and I will gain very much with it and I will have great pleasure; and how can I not do it?" - guard yourself and consider that which you will lose from it in the end of days, as it is many times over that which you will gain now. And the future pain is much longer and bigger than the temporary pleasure. And when you put this into your heart, your hand will cease from doing [it]; as a person does not want a gain that has a greater loss [attached to] it.