Nuhaiq bin Sinan Salmi says: I came to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) and said: I recited the Mufassal surahs in one rak'ah at night. He said: You must have recited quickly like reading poetry or scattering inferior dates. The Qur'an has been detailed so that you may also recite its words slowly and deliberately. Indeed, I know those twenty similar surahs which the Prophet (peace and blessings be upon him) used to combine and recite. According to the compilation of Sayyiduna Ibn Mas'ud (may Allah be pleased with him), it is mentioned to recite Surah Rahman and Surah Najm in one rak'ah, then Surah Dukhan and Surah {عَمَّیَتَسَاءَلُونَ} in one rak'ah.
Hadith Referenceالفتح الربانی / أبواب صفة الصلاة / 1605
Benefits: … The Qur’an has been detailed in this manner so that you too may recite its words with deliberation. Its meaning is that the meanings and concepts of the Noble Qur’an have been explained with utmost clarity, and its rulings and matters have also been established upon solid foundations. Therefore, the reciter should also recite this speech with deliberation and reflection.