He laid the foundation of the language by his book 'Al-Ain' (so called from the letter with which it begins), and by the aid he afforded thereby to Sibawaih, whose master he was, in the composition of his celebrated grammatical work known by the name of 'The Book.
' In the work called 'Al-Ain,' Khalil first arranged the stock of Arabic words, dealing with the organ of speech and the production of sounds, and then dividing the words into classes, the roots of which consisted of one, two, three, four, or five letters. It is still a matter of dispute whether the book 'Al-Ain' was wholly composed by Khalil himself, or completed in course of time by his pupils.
A copy of this celebrated lexicon and work on philology is in the Escurial Library. Khalil also wrote a treatise on prosody, and other works on grammar, and a book on musical intonation. He died A.D. 786, at Busra. 'Poverty,' he said, 'consists not in the want of money, but of soul; and riches are in the mind, not in the purse.'