Using Henna for the fasting Person
Question :
Is it permissible to apply henna to the hair while fasting and praying? I ask this because I heard that henna breaks the fast.
Answer:
This is not correct. For verily applying henna while fasting does not break the fast nor does it have any effect on the fasting person. This is just like kohl, eardrops and eyedrops. All of that does not harm the fasting person nor does it break the fast.
In reference to applying henna during the prayer, I do not know how this question could be possible, as it is not possible for the woman who prays to be applying henna. The questioner probably means to ask does the henna prevent the correctness of the Wudhu' (ablution) if the woman is wearing henna. The answer to this is that this does not prevent the correctness of the Wudhu', because there is nothing in the henna that prevents the water from reaching the skin. It is only coloring. That which has an effect on the Wudhu' is whatever has some substance that prevents the water from reaching the skin. In this case, such a substance must be removed for the ablution to be correct.
Source:
Ash-Shaykh Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 3 Page 264