Musannaf Ibn Abi Shayba 22:214

Khalid ibn Makhlad narrated to us, from Abd al-Rahman ibn Abd al-Aziz, from al-Zuhri, from Urwah ibn al-Zubayr, from al-Miswar ibn Makhramah and Abd al-Rahman ibn Abd al-Qari, they said: We heard Umar ibn al-Khattab saying: The Messenger of Allah (peace be upon him) said: "Indeed, the Quran was revealed in seven ahruf (modes/dialects), so recite from it what is easy."