Sayyiduna Abu Hurayrah Radhiyallahu Anhu reported that Rasulullah Sallallahu Alayhi wa Sallam said, “The best of fasting after Ramadan is fasting in Allah's month of Muharram.” (Muslim)
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Question and Answer:
Q. I have qadaah fasts am I allowed to keep the Muharram fast or do I need to complete my outstanding fasts first?
A. It is better for you to do the qadha fasts. You may do the qadha fast with the Muharram Aashura fasts. According to some Ulama you will receive the thawaab for both.
And Allah Ta’ala Knows Best