In act 1, scene 7, Lady Macbeth encourages Macbeth to keep his promise and kill king Duncan, but is there any textual evidence from the play regarding his earlier oath to kill the king?
Here, for example, Lady Macbeth says that if she had promised to kill her own child, like Macbeth did in the case of killing the king, she certainly would have done so!
I have given suck, and know
How tender 'tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this. (1.7.54–59)