Reference for Kelim 26:7

כָּל מָקוֹם שֶׁאֵין חֶסְרוֹן מְלָאכָה, מַחֲשָׁבָה מְטַמֵּאתָן. וְכָל מָקוֹם שֶׁיֶּשׁ חֶסְרוֹן מְלָאכָה, אֵין מַחֲשָׁבָה מְטַמֵּאתָן, אֶלָּא הָעֻצְבָּה:

Whenever no act is lacking [in order to make vessels fit for use], intentional thought [to use them as they are] renders them [susceptible to becoming] impure. But whenever an act is lacking [in order to make vessels fit for use], intentional thought does not render them impure, except with regard to a [leather] seat-covering [for a wagon or a saddle, which is sometimes used as such even before being fully trimmed or finished].

Jastrow