Shulchan Arukh, Orach Chayim 294/שולחן ערוך·אורח חיים רצ״ד

1The Laws Governing the Recitation of Havdalah in the Shemoneh Esreh, 5 Seifim: We say Havdalah in "He who grants wisdom", and if one erred and did not say Havdalah, he continues his prayers and does not return, since he needs to say havdalah on a cup [of wine]. And if he ate prior to saying havdalah on the cup, he must pray again and recite havdalah in the prayer.

2If one erred and did not say Havdalah in Shmoneh Esrei, and he does not have a cup at night and he believes that even tomorrow he will not have one, must return and pray again.

3If one erred and did not mention Havdalah in the Evening Shemoneh Esreh when Tishah BeAv falls on the day after Shabbos, it is not necessary for him to repeat Shmoneh Esrei, because he will say Havdalah over a cup at the conclusion of Tisha B'Av.

4In a situation where we say one does not go back and repeat Shmoneh Esrei immediately when one finishes the blessing, one does not go back even though he did not begin the following blessing.

5In a situation where we say one does not go back, if one wants to be strict on himself to go back, if he finished his Shmoneh Esrei it is allowed. However, if he had not yet finished the Shmoneh Esrei he is not allowed to go back and repeat.