Most of my answer uses this article as its source (the section on Origin of Vedas). I also took notes from some other articles, but this one includes all of them. Here's the relevant excerpt:
Yajurved 31.7 clearly states that Rigveda, Yajurveda, Samaveda and
Atharvaveda were originated from Him – the ultimate Ishwar who is
Atharvaveda 10.7.20 reiterates the same essence beautifully. It says
that Rik, Yajuh, Sama and Atharva originated from the ultimate Ishwar.
It asks “Which deva (provider ofbliss/knowledge) gave the Vedas?” And
answers that the One who is controlling and maintaining the entire
universe is the creator of Vedas. Atharva is like the mouth of that
Ishwar, Samaveda is like the hair on the skin, Yajurveda is akin to
the heart and Rigveda is the Prana or source.
Shatpath Brahman 188.8.131.52 states that Ishwar, who is present even
beyond the Akash/Sky created the Vedas. The way breath goes out of
body and then comes in, during inception of creation, Ishwar creates
the Vedas and illuminates the world, and in the phase of dissolution
(Pralay), Vedas no more remain in world. However just as a sapling
remains inside the seed, Vedas still remain in knowledge of Ishwar,
Shankaracharya writes in commentary on Geeta 3.15 that Vedas are
actually never created or destroyed. They merely get illuminated and
de-illuminated but remain in Ishwar.
Rigveda 10.190.3 states that the creation remains same in all cycles
and hence even the constitution of the creation – the Vedas – also
remain exactly the same.
One other reference that I saw being referred to was from the Brihadaranyaka Upanishad, though that would not be one of the samhitas. I'm just including it here since I did find it (let me know if you would prefer I not include it):
Brhadaranyaka Upanishad(2. 4. 10) As from a fire kindled with wet
faggot diverse kinds of smoke issue, even so, my dear, the Rg-Veda,
Yajur-Veda, Sama-Veda, Atharvahgirasa, history, mythology, arts,
Upanishads, verses, aphorisms, elucidations and explanations are
(like) the breath of this infinite Reality. They are like the breath
of this (Supreme Self). (translation found here)