Skanda Purana 1.4.3.138_4

1.4.3.138_4

upūpadhmānīyā oṣṭhyāḥ . ^pa ^pha iti paphābhyāṃ prāk
ardhavisargasadṛśaḥ upadhmānīyaḥ . ^ka ^kha iti kakhābhyāṃ
prāk ardhavisargasadṛśo jihvamūlīyaḥ - nāṭyaśāstram 14.11

[Citation:] 'U, p, and the upadhmānīya are labial. Before the sounds pa and pha, the semi-visarga-like sound is the upadhmānīya. Before the sounds ka and kha, the semi-visarga-like sound is the jihvāmūlīya.' — Nāṭyaśāstra 14.11