git how to find commit hash where branch originated from

Pretend I branched from my master branch to a topic branch, and then I did a few commits on my topic branch. Is there a command that tells me the commit hash on the master branch that my topic branch originated from?

Ideally, I wouldn't have to know how many commits I've made ( trying to avoid HEAD^5 ).

I've googled and SO'd around and can't seem to land on the answer. Thanks!

You can use git reflog show --no-abbrev <branch name>. It will output all changes made to the branch, including it's creation, for example (I created branch xxx from master branch):

bdbf21b087de5aa2e78a7d793e035d8bd9ec9629 xxx@{0}: branch: Created from master

Note that this is not very reliable as reflog records can expire (90 days by default), and it seems like there is no 100% reliable way to do this.

use git merge-base master your-branch to find the best common ancestor between two branches (usually the branching point).

The only 100% reliable way to do this is to tag the beginning of your branch when you create it. The accepted answer will not work if you merge commits back to the branch you originated your new branch from. This is sometimes done for example if you are creating a stabilizing release branch and want to merge back your fixes you find during release testing back to master. A common thing to do. If you know you will never merge commits from your new branch back to the originating branch then the accepted answer will work.