The Adityas are considered to be offspring of Aditi and Daityas the offspring of Diti. What is their background?
Aditi appears in the Rig Veda itself. So, that scripture has to be the earliest one.
aditir dyaur aditir antariksham (1),
aditir mAtA sa pitA sa putrah (2),
vishve devA aditih pancha janA (3),
aditir jAtam aditir janitvam (4).
Aditi is the heaven, Aditi is the mid-world (1); Aditi is the mother, the father and the son (2); She is the all-gods (Viswadeva-s); she is the five peoples (3); Aditi is all that is born and what is yet to be born (4).
Rig Veda 1.89.10
Besides this one, there are other Rig Vedic Mantras too where Aditi is mentioned. And, she is mentioned in some Suktas of Krishna Yajur Veda as well.
Rig Veda 4.2.11 mentions Diti and aditi both (unable to give the Sanskrit Mantra for this one):
Lavish on us the finite (diti) and guard the infinte (aditi).
Further references from the Atharva Veda:
dite putrAnAm aditeh akArisham ava (1)
devAnAm anarmanam (2)
teshAm hi dhAm gabhishak samudriyam (3)
nainAn namasA paro asti (4) kaschana (5)
I obtain the protection of Aditi and the children of Diti (1), gods, who are vast, and free from littleness (2); their station is deep like the ocean (3), but there is a power beyond them who can be attained by surrender (4); is it not? (5)
Atharva Veda 7.7.1
So, earliest references for both Diti and Aditi has to be the Rig Veda.