Vaisesika Sūtras
Vaisesika Sūtra 2.1.9
स्पर्शश्च वायोः ॥९॥
sparśaśca vāyōḥ ॥9॥
[sparśaśca = touch is for; vāyōḥ = air;॥]
(Similarly) touch is (the distinguishing mark) of Air.