Mishnah 4 - Chapter 3 - Pirkei Avot

Rabbi chanina ben chachina omer. Hanne'or ballaylah vehammehallech badderech yechidi vehammefanneh libbo levattalah, harei zeh mitchayev benafsho.

Rabbi Hananiah ben Hakinai said: one who wakes up at night, or walks on the way alone and turns his heart to idle matters, behold, this man is mortally guilty.

R. Chanina ben Chachinai says: One who is awake at night, and one who walks on the road alone, [and thinks vain thoughts], and one who turns his heart on vanity is liable for his soul [(because the night is the time of the mazikkim (destructive agents). And one who walks alone on the road is in danger of robbers and other vicissitudes.)]