quit while one's ahead - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Don't try to improve on something that is already accomplished, as in Those drapes we hung are even enough—let's quit while we're ahead. This idiom also implies that further action runs the risk of spoiling something. Also see leave well enough alone.