Musannaf Ibn Abi Shayba 22:206

Sufyan ibn 'Uyaynah narrated to us, from 'Ubayd Allah ibn Abi Yazid, from his father, from Umm Ayyub: She said: The Prophet, may Allah bless him and grant him peace, said: "The Quran was revealed in seven ahruf (modes/dialects). Whichever of them you recite, you have attained correctness."