The 42nd matter from the 70 important matters of fasting:
"Whoever wakes up in the state of major impurity from having relations with his spouse and the time of Fajr has reached him, he is to make the Ghusl after the appearance of the dawn and fast (for the day) based upon the hadeeth of Aaishah and Umm Salamah (may Allaah be pleased with them both) that the Prophet (sallallaahu alayhi wa sallam) said:
"Fajr would reach him while he was in a state of major impurity, from having relations with his family, then he would make the ghusl and fast (for the day)."
(Bukhaaree and Muslim)
Likewise the woman, if she sees that she has become clean from her menses before the Fajr(Dawn) then she is obligated to fast. There is no harm if she delays the ghusl until after the break of dawn. However it is not permissible for her to delay it until the rising of the sun. Rather it is obligatory upon her to make the ghusl and pray before the rising of the sun. Likewise, whoever had a wet dream during the daytime of Ramadaan, there's nothing upon him (i.e no sin. nor any making up of the fast). This is based upon what has been narrated by Abu Hurayrah (may Allaahbe pleased with him) that the Prophet (sallallaahu alayhi wa sallam) stated:
"Indeed Allaah has pardoned my Ummah for that which the self mentions/whispers (inside) as long as it doesn't speak or act in accordance to it (that which was mentioned inside of oneself)."(Bukhaaree and Muslim)
Source: Sab'oon Mas-alah Muhimmah Fee As-Siyaam by Abdullaah ibn Muhammad An-ajmee,Introduction by Al-Imaam Ahmad An-Najmee (may Allaah have mercy upon him) pg. 37
Sisters Upon Al-Istiqaamah