It was narrated from Zirr that a man said to ibn Mas’ood (رضي الله عنه):

How do you read this phrase: ma'in ghairi yasin or asin (in the verse, "water the taste and smell of which are not changed (ma'in ghairi asin)" (Muhammad 47:15))? `Abdullah said to him. Have you read all the Qur'an except this? He said: I recite al-Mufassal in one rak'ah. ‘Abdullah said: Do you recite Qur'an as you recite poetry (i.e., quickly)? I learned the pairs of soorahs that the Messenger of Allah used to recite together, pair by pair, from the beginning of al-Mufassal. And the beginning of al-Mufassal according to Ibn Mas'ood was (Soorat) ar-Rahman.