Musannaf Ibn Abi Shayba 22:312

Thaqafi narrated to us, from Ayyub, from Muhammad, who said: People gathered the Quran until they reached a certain number. Abu Musa wrote to Umar regarding that. Umar wrote back to him: "Indeed, some people have performed it better than others. And perhaps some of those who recite it stand in a position better than the recitation of the other; [so] let them finish what is upon them."