(Devarim 24:1) "and it shall be, if she does not find favor in his eyes, for he found in her a thing of nakedness, (then he shall write her a scroll of divorce.") Beth Shammai say: A man should not divorce his wife unless he find in her a thing of nakedness (i.e., that she had fornicated). And Beth Hillel say: Even if she spoiled his meal, it being written "for he found in her a thing of nakedness."
Shammai Remembered
Sifrei Devarim 269:1