Sahih Bukhari Volume 004, Book 055, Hadith Number 545.

Narated By Abu Salama : Um Salama said, "Um Salaim said, 'O Allah's Apostle! Allah does not refrain from saying the truth! Is it obligatory for a woman to take a bath after she gets nocturnal discharge?' He said, 'Yes, if she notices the water (i.e. discharge).' Um Salama smiled and said, 'Does a woman get discharge?' Allah's Apostle said. 'Then why does a child resemble (its mother)?"