Aristotle - The Organon - index for ANALYTICA PRIORIA Book 1 Part 24

One premiss must be affirmative, one universal

Paragraph 1	Further in every syllogism one of the premisses must be affirmative, and universality must be present:
Paragraph 2	It is clear then that in every syllogism there must be a universal premiss, and that a universal statement is proved only when all the premisses are universal, while a particular statement is proved both from two universal premisses and from one only:
Paragraph 3	It is clear also when a syllogism in general can be made and when it cannot;