stick to one's guns - 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.
Hold fast to a statement, opinion, or course of action, as in The witness stuck to her guns about the exact time she was there. This expression, originally put as stand to one's guns, alluded to a gunner remaining by his post. Its figurative use dates from the mid-1800s.