Mishnah/Seder Nashim/Tractate Sotah/Chapter 1/1
Seder Nashim, Tractate Sotah: Chapter 1, Mishnah 1
Hebrew text 
English Translation 
When you make a warning to a woman - Rabbi Eliezer says: He warns her before two witnesses, and makes her drink on the testimony of one witness or on the testimony of himself.
Rabbi Yehoshua says: He warns her before two witnesses, and makes her drink before two witnesses.