Deuteronomy 27:26:

Cursed be he that confirmeth not all the words of this law to do them. And all the people shall say, Amen.

See also:

Objection: Galatians 3:13 Means That Christ Redeemed Us from the Law, Which Was a Curse, Not the “Curse” in Deuteronomy 28
Objection: Galatians 3:13’s “Curse” Speaks of Spiritual Death, Not Sickness or Poverty
Objection: God Called Deuteronomy 28:15-68 CURSES in the Plural, So Galatians 3:13, Which Says Christ Redeemed Us from THE Curse, Can’t Refer to Those
Objection: There Was Only One Curse (Singular) in Galatians 3:13, So Not Every Sickness Could Be a Curse