Abu Wa'il (may Allah have mercy on him) says that Abdullah (may Allah be pleased with him) said: Have you understood the whole Qur'an except this? He said: I recite the Mufassal surahs in a single rak'ah. Abdullah (may Allah be pleased with him) said: That is like reciting poetry. "People will recite the Qur'an and it will not pass beyond their throats." But when it settles in the heart, it becomes firmly rooted in the heart and also benefits, for "the best prayer is the one with much bowing and prostration." I know the similar surahs of which the Messenger of Allah (peace and blessings be upon him) used to recite two in each rak'ah. Then Abdullah (may Allah be pleased with him) left. After him Alqamah (may Allah have mercy on him) went in; then we went to him, and he said: He has informed me about them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4689