Rashi on Deuteronomy 23:10/רש"י על דברים כ״ג:י׳

1כי תצא וגו׳ ונשמרת WHEN [THE HOST] GOETH FORTH [AGAINST THINE ENEMIES], THEN KEEP THEE [FROM EVERY EVIL THING] — because Satan accuses men in time of danger (Jerusalem Talmud Shabbat 2:6; Midrash Tanchuma, Vayigash 1 on Genesis 42:4; cf. Rashi on that verse and our Note thereon).