Praise be to Allaah.
If the woman who is bleeding in nifaas becomes pure (i.e.,
the bleeding stops) within forty days and she fasts for a few days, then the
bleeding comes back within forty days, then her fast is valid, but she has
to stop praying and fasting during the days on which the bleeding comes back
– because it is (still) nifaas – until she becomes pure or the forty days
end. When forty days have ended, she has to do ghusl, even if she has not
seen the tuhr (white discharge signalling the end of menses or nifaas),
because forty days marks the end of nifaas according to the more sound of
the two scholarly opinions. After that, she has to do wudoo’ at the time of
every prayer, until the bleeding stops, as the Prophet (peace and
blessings of Allaah be upon him) instructed the woman who was suffering from
istihaadah (non-menstrual vaginal bleeding). And her husband may be intimate
with her after forty days, even if she has not seen the tuhr, because the
bleeding in this case is irregular bleeding which does not prevent her from
praying and fasting, and does not prevent her husband from being intimate
with his wife.
But if the bleeding after forty days coincides with the usual time of her menses, then she should stop praying and fasting and regard it as menses. And Allaah is the Source of Strength. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him). Fataawa Islamiyyah (2/146).