Jaiminīya Upaniṣad Brāhmaṇa 3.18.7

3.18.7

sa eva tena vasiṣṭhasyaikastomabhāgānumantreṇānumantrayeta bahur eva prajayā paśubhiḥ prajāyate
iyaṃ tv eva sthitir om ity evānumantrayeta /
caturthe 'nuvāke caturthaḥ khaṇḍaḥ //

If he should accompany with that single accompanying formula of the share-of-the-praise-chant belonging to Vasiṣṭha, he is abundantly propagated with offspring and cattle.
However, this settled rule is: one should accompany with 'om' alone.
Fourth subsection in the fourth lesson.