c. In the older language, the numerals for 5 and upward are sometimes used in the nom.-acc. form (or as if indeclinably) with other cases also: thus, páñca kṛṣṭíṣu among the five races; saptá rṣīṇām of seven bards; sahásram ṛ́ṣibhiḥ with a thousand bards; çatám pūrbhíḥ with a hundred strongholds. Sporadic instances of a like kind are also met with later.
Source: wiktionary