Imam Raza (as) said (part of a lengthy narration):
"When the new moon of the month of Ramadhan would come up, the noor (light) of Fatema Zahra (sa) would over come the new moon and the moon would be hidden and when She (sa) would go then the moon would become visible."
[Source: Behar al-Anwar, Vol. 43. Pg. 57]