Musar for Berakhot 7:15

Even though a person be guiltless and completely righteous he should always be fearful that a sin might come to him. And so have we found in the case of Jacob that even though the Holy One, Blessed be He, had said to him "And I will keep Thee withersoever thou goest" (Gen. 28:15), Jacob was nevertheless afraid that he had sinned (and lost the divine promise). And so did David say "If I had not believed that I would see the goodness of the Lord, in the land of the living!" (Ps. 27:13). Even though David knew that he was a perfectly pious man he feared that sin might prevent him from receiving the pleasant reward of the righteous (Berakoth 4a).