What must I do after uttering the oath of divorce in a state of anger for the first time?

Anger does not prevent the occurrence of divorce as long as the husband is aware of what he is saying. And swearing by divorce causes divorce to occur upon breaking the oath, and this is the view of the majority of scholars. Sheikh Al-Islam Ibn Taymiyyah holds that its ruling is the same as swearing by Allah, so an expiation for an oath becomes obligatory, and divorce does not occur if the intention was to threaten or emphasize. Therefore, if you broke your oath of divorce while you were aware, divorce has occurred. If it was the first or second divorce, you may take her back before her waiting period (iddah) expires. If it was the third, she becomes irrevocably separated from you (baynunah kubra), and she is not lawful for you until she marries another husband and he consummates the marriage with her, then he divorces her or dies, and her waiting period ends. The lawful oath is by Allah Almighty, and swearing by divorce is among the oaths of the dissolute.