Musnad Ahmad 26:449

'Abd al-Samad narrated to us, Mahdi narrated to us, Wasil narrated to us, from Abu Wa'il, from 'Abdullah, who said: "I indeed memorize the pairs that the Messenger of Allah, may Allah bless him and grant him peace, used to pair together: eighteen surahs from the Mufassal, and two surahs from Al-Ha Mim."