I think that I know which lines you are talking about. I think that you are talking about something that comes from Act III, Scene 5. The lines I have in mind are spoken by Phebe, just as you said. And they seem to have the same sort of meaning that you are looking for. Here is the line I think you are talking about:
Dead shepherd, now I find thy saw of might:
'Who ever lov'd that lov'd not at first sight?'
This is spoken about halfway through the scene, around Line 90. The "dead shepherd" that she talks about here is Christopher Marlowe, who was a friend of Shakespeare's. The "saw" is not a saw for cutting things, but a saying.