Shiva Purana
तत्राकारो ऋगभवदुकारो यजुरव्ययः ॥ मकारस्साम संजातो नादस्त्वाथर्वणी श्रुतिः ॥ ७ ॥
There the letter “a” became the Ṛgveda, the letter “u” the Yajurveda, the letter “ma” the Sāmaveda and the Nāda the Atharvaveda.
english translation
tatrAkAro RgabhavadukAro yajuravyayaH ॥ makArassAma saMjAto nAdastvAtharvaNI zrutiH ॥ 7 ॥
hk transliteration by Sanscript