The Messenger of Allah ﷺ said, "When she sees the moisture, let her perform the ritual bath."
قال رسول الله ﷺ: إذا رأت الرطب فلتغتسل.
No references are available.
Ishaq narrated it in his Musnad (4/168-169, no. 1951). He said, Muhammad bin Bakr told us, Ibn Jurayj informed us, Ibn Khuthaym informed me, that Sulayman bin Atiq told him that a woman came to Umm Salama and said, "I saw in a dream as if so-and-so was marrying me." Umm Salama mentioned this to the Messenger of Allah ﷺ, and he said, then he related the hadith.Its isnad is hasan because of Ibn Khuthaym, who is Abdullah bin Uthman bin Khuthaym, for he is one whose hadith is hasan. Sulayman bin Atiq al-Madani is among the men of Muslim, and Nasai deemed him trustworthy. This hadith was cited by Hafiz in al-Matalib (1/117, no. 204), and he passed over it in silence.