Korah's rebellion; Aaron's staff buds
Korah, Dathan, and Abiram led 250 leaders against Moses and Aaron; the earth swallowed the rebels, and Aaron's staff budded to confirm God's choice.
On the next day, Moses went into the Tent of the Testimony; and behold, Aaron’s rod for the house of Levi had sprouted, budded, produced blossoms, and bore ripe almonds.
Numbers 17:8 (World English Bible)
Korah, a Levite, wanted the priesthood for himself, and he rallied others who resented Moses' leadership. God judged them: the ground opened under Dathan and Abiram, and fire consumed the 250 men offering incense.
To settle the question for good, each tribe's leader placed a staff before the ark overnight. By morning, Aaron's had sprouted, blossomed, and produced ripe almonds.
Dating notes
Not dated in Scripture. Ussher suggests the events of Numbers 15–19 happened in the latter half of the second year; the placement is approximate.