How to know from which yum repository a package has been installed?

If all else fails, you can inspect the yumdb manually. It’s located in /var/lib/yum/yumdb and contains detailed information on every installed package. You’ll be particularly interested in from_repo. For example, for the bind-utils package:

# for i in /var/lib/yum/yumdb/b/*bind-utils*/*; do echo $i: `cat $i`; done
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/checksum_data: 39f7840f93d3d76825a9e8da79cfe0366f7fad68f018a273477aee62cccfa3f4
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/checksum_type: sha256
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/command_line: install bind-utils
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/from_repo: updates
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/from_repo_revision: 1397654451
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/from_repo_timestamp: 1397654759
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/installed_by: 0
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/reason: user
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/releasever: 6

To get just the information you want:

# cat /var/lib/yum/yumdb/b/*bind-utils*/from_repo
updates

Later versions will provide even more data; for instance in EL 7 the exact mirror and URL from which the package was downloaded is stored here.