Who could show me an example of the ending -κειμεν?

Well, this is a “late” (i.e. Koine) variant of the 1pl pluperfect active ending -κεμεν, as in “we had untied, ἐλελύ-κεμεν”. So you won’t likely get a Classical form.

The earliest instance I find is in Aristotle, Metaphysica 1041a:

καίτοι κἂν εἰ μὴ ἑωράκειμεν τὰ ἄστρα, οὐδὲν ἂν ἧττον, οἶμαι, ἦσαν οὐσίαι ἀΐδιοι παρ’ ἃς ἡμεῖς ᾔδειμεν

However, I presume that even if we had never seen the stars, none the less there would be eternal substances besides those which we knew

After that, it’s Josephus, Philo, Plutarch and Appian.