A’mash said that he heard Abu Wail say that a man asked Abdullah about the word “Ghairi Aasin” or “Yasin”. He asked, “Have you read the whole Qur’an besides it.” He said, “Yes!” Abdullah ibn Mas’ud (RA) remarked, “Some people recite the Qur’an as though they scatter rotten dates. They do not get the Qur’an below their throats (meaning it has no effect on them). I know of such mutashbihaat surah (whose definite meanings are not known) which the Prophet (SAW) recited together.”
[Ahmed3607,8 4995, Muslim 822, Nisai 1000]
The subnarattor said that they asked Alqamah, so he asked lbn Mas’ud about these surah. So, he said, “They are twenty surah of Mufassal which the Prophet (SAW) recited two together in one raka’ah.”