After Macbeth kills Duncan, he goes to Lady Macbeth and is concerned about not being able to say "Amen". What is her advice to him?

After Macbeth kills Duncan, he is indeed filled with guilt and fear, and tells Lady Macbeth that he was unable to say "Amen" when he heard a voice say "God bless us".

Lady Macbeth's advice to him is pragmatic and cold. She tells him that thinking about what they've done will make them crazy, so it's better not to dwell on it. She says, "These deeds must not be thought after these ways; so, it will make us mad."

She also tells him to wash the blood off his hands, saying "A little water clears us of this deed." This is her way of trying to minimize the gravity of their actions and to convince both Macbeth and herself that they can move on from this without any guilt or repercussions.