In Act 2 there is a scene where Macbeth just killed Duncan and is heading back to lady Macbeth covered in blood. They have a conversation about the incident that took place and suggest that the fact that he could not return the weapon was because he felt guilty. He also kept muttering about all the blood on his hands, coming from the king, on the daggers.
He imagined hearing noises when he first comes in imagining people in the castle that could be witness to his crime. He also said that when he was committing the crime, he thought he heard someone say “Sleep no more... Macbeth has murdered sleep”. This was also an indication of guilt from his conscience .
I hope this helps, Regards.