Abū Dāwūd al-Sinjī (1) related: “I heard al-Aṣmaʿī say,
‘The greatest danger I fear for a seeker of knowledge who is ignorant of nahw (Arabic grammar) is that he might fall under the Prophet’s warning: “Whoever deliberately attributes a lie to me, let him take his seat in the Fire.” (2)’

Naṣr al-Jahḍamī said, “Al-Aṣmaʿī was as cautious about explaining a hadith as he was about explaining the Quran.” (3)

Al-Mubarrad remarked, “In matters of language al-Aṣmaʿī was an ocean; we know no one his equal. Abū Zayd, however, was more skilled in grammar than he.” (4)

Someone told Abū Nuwās, “Al-Aṣmaʿī and Abū ʿUbaydah have been summoned to Hārūn al-Rashīd.”
He replied, “If they give Abū ʿUbaydah access to his manuscripts, he will recite to them the lore of those who came before and those who will come after; as for al-Aṣmaʿī, he is a nightingale who will charm them with his songs.” (5)

Abū ʿAynāʾ reported that al-Aṣmaʿī said:
“I entered, together with Abū ʿUbaydah, upon al-Faḍl ibn al-Rabīʿ. He asked, ‘Al-Aṣmaʿī, how many volumes is your book on horses?’
I answered, ‘A single volume.’”