Musannaf Ibn Abi Shayba 22:248

Waki’ narrated to us, from Sufyan, from Ubayd al-Maktab, who said: Mujahid was asked about two men, one of whom recited al-Baqarah, and the other recited al-Baqarah and Ali ‘Imran, while their bowing, prostration, and sitting were equal. Which of them is better? He said: "The one who recited al-Baqarah." Then Mujahid recited: "And [it is] a Qur'an which We have separated [by intervals] that you might recite it to the people over a period, and We have sent it down progressively."