Mishnah for Nedarim 78:2

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.