Musnad Abu Ya'la al-Mawsili 152:154

Al-A’mash narrated to us, from Shaqiq, who said: A man came to Abdullah and said, "O Abu Abd al-Rahman, how do you recite this verse: 'from water not stagnant'?" Abdullah said to him, "Have you counted all the Quran except for this? Verily, I recite the Mufassal in one rak'ah." Abdullah said to him, "This is like the reciting of poetry. Verily, the best of prayer is the bowing and the prostration. And there will be people who recite the Quran, but it will not pass beyond their collarbones. But if it is recited and it settles in the heart, it benefits. Verily, I know the pairs that the Messenger of Allah (peace be upon him) used to recite in every rak'ah." Then he stood up and entered [his house]. Alqamah went in to him and said, "Ask him for us about the pairs that the Messenger of Allah (peace be upon him) used to recite." He said: Then he came out to us and said, "Twenty surahs from the Mufassal, according to the arrangement of Abdullah."