In the mantra-s , the beginning of ātman is elided when the affix of the Instrumental -3rd case singular follows.,
आङ् is the name of the Instrumental singular affix, given by ancient Grammarians. Thus त्मना देवेभ्यः, त्मना सोमेषु ॥ Why in the Mantras? Observe आत्मनो कृतम् ॥ Why in the Instrumental Singular? यदात्मन स्तन्नो वरिष्ठा ॥
Vart:- The elision of आ of 'atman' is found in other cases also, but not so frequently as in the Instrumental, as त्मन्यासमञ्जत मह्यम् ॥,