Skandapurana 16.17.146

16.17.146

tajjāpyaṃ paramaṃ proktaṃ svayaṃ devena bhānunā ..
jāpyasya viniyogo'sya lakṣaṇaṃ ca nibodha me ..
stobhasāraṃ śvāsalīnamoṃkārādi smṛtaṃ budhaiḥ ..146..

That supreme text to be muttered was proclaimed by the Sun God himself; / learn from me the ritual application and characteristics of this recitation: / the essence of stobhas, merging with the breath, beginning with the syllable Oṃ, is remembered by the wise.