The claim: "The Qur'an says 'establish the prayer' but does not say how (number of cycles, sittings). So practice needs a source outside the Qur'an (hadith); hence the Qur'an alone is not enough."
The Qur'an gives the prayer's frame
- Facing the qiblah: turn toward the Sacred Mosque (2:144).
- Standing & recitation: the Qur'an is recited in prayer (17:78-79).
- Bowing & prostration: "Bow and prostrate" (22:77; 2:43).
- Khushūʿ: the believers are humble in prayer (23:1-2).
- Time: prayer is "a decree at fixed times" — "kitāban mawqūtā" (4:103); also the "middle prayer" (2:238).
It even gives some details
Contrary to the claim, the Qur'an does enter into some specifics:
- The shortened and fear-prayer arrangement is described in detail (4:101-102).
- The Friday prayer: when called, leave trade and hasten to it (62:9-10).
So the premise "the Qur'an gives no detail of prayer" is false.
An honest limit
Fact: the number of rakʿahs and the set phrases are not in the Qur'an. Inference: "no number means the source is incomplete." This second part is an interpretation — the Qur'an calls itself "complete" (6:38; → Does the Qur'an call itself sufficient?). Whether to treat the rakʿah pattern as "mass-transmitted practice" or "a detail left open" is also interpretation; we leave both as they are. What is clear: the Qur'an gives the prayer's frame and many of its details.
Source: Qur'anic verses (M. Okuyan meal). Presented with a text/interpretation distinction.