There was no mention of mahr at the time of nikah.

Question: There was no mention of mahr at the time of nikah. Is the nikah valid?

Answer: Yes it is valid. That mahr will be wajib for the groom which will be mar-e-mith. Mahr mith is that mahr which was given to a woman of the girls fathers family, whom are equal in beauty, age etc to the girl getting married. Whatever that girl received when she got married, this girl will receive the same amount of mahr.