Sutra:
13. iththai vEthankaLum rishikaLum AzhvArkaLum AcAryarkaLum virumbinArkaL.

Meaning:
This Thiruvashtakshara mahA manthra is loved and supported by the Vedas, the Rishis that came to explain the Vedas, the Azhvars as well as our Acharyas.

Sri PBA Swami's Sarartha Deepikai:
The manthras other than Thirumanthra came to be supported by kudrushtis because of their incompleteness. Conversely we would expect that because of its completeness the Thirumanthra would be greatly supported by the true followers of Sriman Narayana. This is being shown in this sUtra.

Here the Vedas means the height of the Vedas which are the Upanishads. In many Upanishads, in the Ithihasas and Puranas written by great Rishis, in the divine works of Azhvars, and in the knowledge imparting works of our Acharyas, the greatness of the Thirumanthra is well spoken about.