Ibn al-Ṣalāḥ (d. 1154 CE) wrote a critique on chapter V.17 of al-Fārābī’s commentary on the Almagest, which is preserved in a unique MS in Mashhad. More than half of its text consists of literal quotations of al-Fārābī’s commentary, which previously was considered to be lost. A commentary on books IX–XIII of the Almagest, handed down anonymously in two MSS and recently discovered was attributed to al-Fārābī.1 This assumption can now be tested on a more secure basis. Al-Fārābī’s commentary was evidently not limited to the cosmological parts of the Almagest, but reached far into the mathematical parts, and makes the existence of books IX– XIII more likely. Furthermore, comments are verbose and larger in size than Ptolemy’s corresponding text, especially in the geometrical proofs. Finally, the quoted passages of al-Fārābī concern the calculation of parallaxes. In the two extant Arabic translations of the Almagest Greek parallaxis ist translated by ikhtilāf al-manẓar (“difference of the aspect”), and this became the standard term in astronomical works. But in the passages of al-Fārābī, the uncommon expression inḥirāf al-manẓar (“inclination of the aspect”) is consistently used instead. Parallaxis occurs only in one chapter of books IX–XIII and the anonymous commentary uses the expression inḥirāf al-manẓar. This striking common feature corroborates the attribution of the commentary of books IX–XIII to al-Fārābī. An edition and translation of the texts is in preparation.