You can say the dua al istikharah after you have reached the decision, and you do just the dua by itself, without the prayer.

I have almost made up my mind to take a step in my life, And of course i want to pray salaat al-Istikhaarah. But actually i am on my menses. Can i take the decision and do the step i am thinking of doing and pray istikhara in sha Allah as soon as they are over? Or do i have to wait?

Prayer | November 8, 2015 3:40 am
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In a book of hafiz Al Asqalani, book #2, chapter 7, ”Bab Sifat Salat”, hadith 271: mother Aisha r.a. reported that Prophet saw prohibited muslims sit like a satan. What does it mean ”sit like a satan”??? its part of a longer hadith.
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If i start the first rakat by not seeking refuge from Allah & not bismillah. Instead i start with Allhumdu straight away will my prayer be invalid?