Accounts of the Companions’ consensus that the Quran is uncreated:

It is reported from Ali—may Allah be pleased with him—that on the day of Siffin he said, “I did not appoint a created being as arbiter; rather, I appointed the Quran as arbiter.” The Companions of the Messenger of Allah ﷺ were with him, while Muawiyah had more than he did. Thus, this was a consensus established openly and widely, with that generation passing away without any disagreement or objection.

The same is reported from Ibn Abbas, Ibn Umar, and Ibn Masud.