While commenting on sermon 91 of Imām ‘Alī (a) – as recorded in Nahj al-Balāgha – the Mu’tazlī scholar Ibn Abī al-Ḥadīd (d. 656 AH / 1258 CE) is left in awe with the way the Imām (a) describes the angels. Before he begins to comment on the actual sermon, he writes:
This is an appropriate place to cite the idiom: When the river of Allah flows, the river of Ma’qil is of no use. In other words, when this divine speech and sacred words are present, the eloquence of the Arabs is of no use. The relationship of their eloquent speech to his (‘Alī’s) speech is like the relationship of dust to pure gold. Even if we were to assume that the Arabs were capable of producing such eloquent speech, where did they have the substance for them to then use words to convey it? Where did the people from the era of ignorance, or rather, even the companions of the Messenger of Allah (p) know about such ambiguous and celestial topics, that they would have been able to describe them?
As for people from the days of ignorance, their eloquence would be expressed in their descriptions of camels, horses, wild donkeys, bulls, the mountains, deserts and so on. As for the companions, then those who were known for their eloquence, their eloquent words do not exceed two or three lines. Their words were often admonitions regarding death, or condemnation of the world, or motivational words during war and battles.
As for the description regarding the angels, their qualities, their forms, their worships, their glorification, their recognition of their Creator, and their love towards Him, and their bewilderment towards Him, and other related matters which have been mentioned in this section in detail, then this information was not known amongst the companions in such detail.
Yes, perhaps they knew a thing or two regarding the angels through what they had heard from the Holy Qurān, but not divided or organized in the way it is in this sermon. As for those who did have knowledge of these details, like ‘Abdullah b. Salām, and Umayyah b. Abī al-Ṣalt and a few others, then they did not have the ability to produce or convey it with such eloquence.
It is thus proven that these details, conveyed in such an eloquent way, was something ‘Alī (a) alone possessed. I swear, if a sensible person was to ponder over this sermon, his skin will get goosebumps, and his heart will begin to beat fast, and he will sense the greatness of Allah (swt) deep within himself.
Source: Sharh of Nahj al-Balāgha by Ibn Abī al-Ḥadīd, volume 6, page 425-426
 Āyatullah Montazerī (d. 2009) explains that people of Basra were short on water, so ‘Umar – the 2nd caliph – asked Abū Mūsa al-Ash’arī to ask Ma’qil b. Yasār to build a dam in order to channel the high water levels and form a river that would pass through Basra. This was an important project of the time and the river became attributed to Ma’qil. Some event must have taken place – which we are unaware of – for this idiom to be formed: ‘When the river of Allah flows, the river of Ma’qil is of no use.’ Ibn Abī al-Ḥadīd uses this idiom to compare the eloquence of the Imam and the rest of the Arabs of the time.
 Āyatullah Montazerī says: As for the claim made by Ibn Abī al-Ḥadīd that ‘Abdullah b. Salām, Umayyah b. Abī al-Ṣalt and others possessed this knowledge – this is just a mere claim, because this knowledge concerns the unseen, and only the Prophet (p) and the Imams (a) possessed such knowledge. Others did not have any access to it. – Dars-hayi az Nahj al-Balāgha, volume 3, lesson 120, page 543.