Aulus Gellius, Noctes Atticae 1.16
“Milli passum” dixit pro “mille passibus” et “uno milli nummum” pro “unis mille nummis” aperteque ostendit “mille” et vocabulum esse et singulari numero dici eiusque plurativum esse “milia” et casum etiam capere ablativum.
He said milli passum instead of mille passibus and uno milli nummum instead of unis mille nummis and clearly demonstrated mille to be a noun, that it is used in the singular, that its plural is milia, and also that it has an ablative case.
He calls attention to the fact that Latin mille doesn’t quite act like Greek χίλιοι chilioe, which is usually an adjective, but more like the noun χιλιάς chilias, which is less common. In English, too, “thousand” acts noun-like, unlike most of the smaller numbers which are more adjectival.