Yoreh Deah Siman 267 Seif 11

יא מִי שֶּׁיֵּשׁ לוֹ שִׁפְחָה וַעֲדַיִן לֹא טָבְלָה לְשֵׁם עַבְדוּת, אָסוּר לְאַחֵר לְהַטְבִּילָהּ בְּלֹא רְשׁוּת הָאָדוֹן.

 

He who has a maidservant and has still not immersed her for slavehood, it is forbidden to do so afterwards (before the fact) without permission from her master.