Macbeth often uses an image of a serpent or a snake. (For background, reread the play through Act 3, scene 4 to note those references.) The lines you seek are in Act 3. scene 4:
Macbeth asks Banquo's murderers,"But Banquo's safe?"
Ay, my good lord: safe in a ditch he bides,
With twenty trenched gashes on his head;
The least a death to nature.
Thanks for that:
There the grown serpent lies; the worm that's fled
Hath nature that in time will venom breed,
No teeth for the present....
When you tease out the meaning of the lines, notice
1. "There the grown serpent lies."
2. Untangle the grammar: "the worm that has fled has a nature that in time will breed venom...."
3. What are we to do with the comma after "breed"?