As for his 'Kitab,' or 'Book,' composed by him on that subject, it has never had its equal. The great philologist and grammarian, Al-Jahiz, said of the book of Sibawaih, that none like it had ever been written on grammar, and that all writers on this subject who had succeeded him had borrowed from it.
When Al-Kisai was tutor to the prince Al-Amin, son of Harun-ar-Rashid, Sibawaih came to Baghdad, and the two great grammarians (Sibawaih, the chief of the school of Busra, and Al-Kisai, chief of the school of Kufa) had a long dispute about a certain expression of Arabic speech, and an Arab of the desert was called in to arbitrate between them.
The man first decided in favour of Sibawaih, but when the question was put in another form, the Bedouin asserted that Kisai was right. As Sibawaih considered that he had been unjustly treated in the matter, he left Baghdad for good.
The year of his death has been given differently by various authors, the earliest date being A.D. 787, and the latest A.D. 809.