Who is the man mentioned in the Hadith who recited al-Mufassal in one rak'ah, and what is meant by the words "al-Mufassal" and "an-Naza'ir" mentioned therein, and what are the Surahs of an-Naza'ir?

The hadith was narrated by Bukhari and Muslim and others, and Muslim explicitly mentioned the man's name as Nahik ibn Sinan. Al-Mufassal refers to the surahs of the Qur'an from Surah Qaf to Surah An-Nas, and they were named as such due to the frequent separation between their surahs by the Basmala. As for "I knew the pairs (an-naza'ir)," it refers to the surahs that are similar in meaning, such as those containing admonition, wisdom, or stories. Ibn Mas'ud enumerated some of these paired surahs that the Prophet used to recite two of them in one rak'ah.