Genesis 6:19

And of every living thing of all flesh, two of every sort shalt thou bring into the ark, to keep them alive with thee; they shall be male and female.

Cross-reference

Genesis 7:2 expands this command, specifying seven pairs of clean animals instead of just two.

Genesis 7:3 extends the command to birds, also requiring seven pairs.

Genesis 7:8 lists the animals entering the ark, fulfilling this command.

Genesis 7:9 explicitly states animals entered two by two as commanded here.

Genesis 7:15 repeats the two-by-two entry of all flesh, confirming this command.

Genesis 7:16 notes the entry of male and female as commanded, with God shutting the ark.

Genesis 8:17 reverses this command, instructing Noah to release the animals from the ark.