Midrash for Numbers 5:16

(Bamidbar 5:16) "And the Cohein shall draw her near": whence it is derived that the draught is not given to two sotahs together. "and he shall stand her": He shall not stand with her, her manservant and her maidservants, because she tends to be callous in their presence. "before the L-rd": at the gates of Nikanor (opposite the entrance to the sanctuary), whence they stated: The head of the watch would stand those who were tamei at the gates of Nikanor.