STM32F4-Discovery board RTC
The User Guide for your board recommends a MC306-G-06Q-32.768.
I referred to the schematic on page 28:

Don't forget to also fit R21, R22, C16 &amp; C27 too.  

If you choose a different crystal to use, make sure you check that crystal's datasheet for the appropriate capacitance values to use for C16 &amp; C27.