Skandaswami (7th Century) 's commentary on Rigveda
Recalling what I read in the preface of Rigveda book from Gayatri Pariwar (you can read this paragraph at page number 19:
This is in Hindi language, the English translation of first couple of sentences would be:
Who did write the very first commentary on Rigveda and when? This is not sure but among the commentaries available today, the first/oldest one is written by Skandaswami......The time period should be around 652 AD....
I have searched to verify it from other sources:
Found a research paper named Skandasvamin and the Vedic canon, quoting from Abstract (the full text is not freely available):
The example of Skandasvāmin (7 th century CE), who composed the first commentary of a bhāsya type on the Rgveda, attests an original view of the hymns. His reflexions on the nature of the mantras and their relation with the ritual shed light on his perception of the Vedic « canon ».
Hindupedia article on Skandasvami:
Skandasvāmin lived in A. D. 630. Ancient scriptures in Sanskrit, especially the Vedas, are difficult to understand because of their archaic language and recondite ritual system. A commentary written by scholars coming in the Vedic tradition will be very helpful in unraveling their mysteries. Skandasvāmin is the earliest of the commentators on the Ṛgveda known to us now, and whose commentary, though incomplete as extant now, is available
I recommend to read the introduction in English from the book The Riksmahita with Bhasya on Sankdasvamin and Dipkia of Venkatamadhavarya by The Superintendent, Government Press Trivendrum, for more information on Sknadasvami's commentary.
A history of Vedic Literature Vol I, part II, Commentators on Vedas which starts with first topic Skandasvamin and says the same that Skandasvami (A.D. 630) is the oldest known commentator on Vedas:
International Journal of Academic Research and Development Volume 2; Issue 2; March 2017; Page No. 132-136, ISSN: 2455-4197
says Skandaswami is considered/believed to be the oldest among commentators of Vedas.
- स्कन्दस्वामी (638ई .) –
वैदिक भाष्यकारों की गणना में स्कन्दस्वामी को सबसे प्राचीन माना जाता है
So, the conclusion is that Skandaswami, who lived in 7th century, is the oldest known commentators on Vedas.