In act 4, scene 1 of Shakespeare's The Tempest, Prospero apologizes to Ferdinand for treating him harshly, and gives permission for Ferdinand to marry his daughter, Miranda.

PROSPERO: Then, as my gift and thine own acquisition
Worthily purchased take my daughter (4.1.14–15).

Prospero has cared for Miranda on his own for twelve years, since she was three years old, and he's very protective of her.

PROSPERO: I have done nothing but in care of thee . . . (1.2.19)

So when Prospero gives his permission for Ferdinand to marry Miranda, Prospero imposes some conditions on Ferdinand, which he puts to Ferdinand in the form of a threat.

PROSPERO: But
If thou dost break her virgin-knot before
All sanctimonious ceremonies may
With full and holy rite be ministered,
No sweet aspersion shall the heavens let fall
To make this contract grow; but barren hate,
Sour-eyed disdain and discord shall bestrew
The union of your bed with weeds so loathly
That you shall hate it both (4.1.15–23).

In other words, if Ferdinand has sexual relations with Miranda before the wedding, from that moment Prospero will make their life together miserable.

Prospero clearly overstates his threat, so it's debatable as to whether he actually means what he says. Prospero might simply be trying to impress Ferdinand with how protective he is of Miranda and the importance that Prospero places on the marriage and on Miranda's happiness.

Nevertheless, whether Ferdinand believes that Prospero will carry out his threat or not, Ferdinand isn't going to take any chances, and he agrees to abide by Prospero's wishes.

FERDINAND: As I hope
For quiet days, fair issue and long life
With such love as ’tis now, the murkiest den,
The most opportune place, the strong'st suggestion
Our worser genius can, shall never melt
Mine honour into lust to take away
The edge of that day's celebration . . . (4.1.25–31)

On his honor, says Ferdinand, there is nothing in the world that will make him break his word to Prospero that he will honor Prospero's wishes. Prospero takes Ferdinand at his word.

PROSPERO: Fairly spoke.
Sit then, and talk with her. She is thine own (4.1.34–35).

Prospero and Ferdinand never speak about it again.