It was given to Noah, BY GOD, when God told him to bring the animals into the Ark.

God specifically instructs Noah.

Clean animals were pairs of 7 each.

Unclean animals pairs of 2 each.

Why make a distinction of clean and unclean, if God didn't wish it to be followed?

Genesis 7:2-5 King James Version (KJV)
2 Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female.

3 Of fowls also of the air by sevens, the male and the female; to keep seed alive upon the face of all the earth.

4 For yet seven days, and I will cause it to rain upon the earth forty days and forty nights; and every living substance that I have made will I destroy from off the face of the earth.

5 And Noah did according unto all that the Lord commanded him.