TgO Num1:53 :וְיִטְרוּן לֵיוָאֵי יָת מַטְרַת מַשכְנָא דְסָהֲדוּתָא‏ : the Levites are to keep the watch over the tent of testimony
See the entire entry for sāhdū, sāhdūṯā n.f. testimony