Sahih Bukhari Volume 009, Book 086, Hadith Number 091.

Narrated By Muhammad bin 'Ali : 'Ali was told that Ibn 'Abbas did not see any harm in the Mut'a marriage. 'Ali said, "Allah's Apostle forbade the Mut'a marriage on the Day of the battle of Khaibar and he forbade the eating of donkey's meat." Some people said, "If one, by a tricky way, marries temporarily, his marriage is illegal." Others said, "The marriage is valid but its condition is illegal."