Allah says in the Quran (to Ayyub A.S.):
‘Take a small bunch of grass in your hand, and strike [her] with that so as not to break your oath.’ We found him patient in adversity; an excellent servant! He, too, always turned to God. (38:44)
According to what I have read of Ibn Kathir and other tafasir, they say that the Prophet Ayyub (AS) made a promise with Allah that he would punish his wife with 100 lashes because she did something wrong.
However, I couldn't find what the source of this story was. Is it a hadith? If so, from where?