From Sahih Bukhari (Hadith 6748):
Narrated Ibn `Umar:A man and his wife had a case of Lian (or Mula'ana) during the lifetime of the Prophet (ﷺ) and the man denied the paternity of her child. The Prophet (ﷺ) gave his verdict for their separation (divorce) and then the child was regarded as belonging to the wife only
Reference: Bukhari 6748