There are quite a few quotations found in Macbeth which can be used to show the mental deterioration of Macbeth. The following quotes are found in Act II, scenes one and two.
In the following passage, Macbeth is preparing to murder Duncan. Up to this point, Macbeth's mental stability has yet to be questioned. Macbeth's seeing of the hallucination proves his mental faltering. He even acknowledges his mental sickness (thorough the description of his "heat oppressed brain").
Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee.
I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight? Or art thou but(45)
A dagger of the mind, a false creation,
Proceeding from the heat oppressed brain?
In the following two passages, from scene two, Macbeth's acknowledgement of his inability to say "Amen" and dream are signs of his mental deterioration. Given that these two things are the result of his murder of Duncan, his inability to pray and dream are caused by the enormous guilt the murder has placed upon him. As a reaction, his guilt manifests in his inability to do certain things.
But wherefore could not I pronounce “Amen”?
I had most need of blessing, and “Amen”
Stuck in my throat.
Still it cried, “Sleep no more!” to all the house;
“Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more. Macbeth shall sleep no more.”
Macbeth's mental deterioration is apparent. The hallucinations, his inability to sleep, and his inability to pray show the direct impact his actions have upon his mind. His guilt is far to strong for his mind to deal with and, therefore, his mind begins to falter.