Musar for Ketubot 59:17

Perhaps you will say: behold we see that the sages everywhere obligated a man to guard himself well and not put himself in danger even if he is a righteous person with many merits. For instance: "everything is in the hands of heaven except colds and heatstrokes" (Ketubot 30a) and the Torah says "you shall guard yourselves very carefully" (Devarim 4:15). Hence one should not decide to "trust in G-d" in all situations, and in the Talmud Yerushalmi (Berachot 3, see also Chulin 142a) they said: "even when performing a Mitzva!".