Is it permissible to perform the Istikhaarah (guidance-seeking) prayer after the Fajr (dawn) or the ‘Asr (afternoon) prayers? If it is impermissible, why is this so?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Scholars of the four schools of Fiqh unanimously agree that the Istikhaarah prayer is impermissible at times when performing (voluntary) prayer is forbidden, including both those who consider a reason-based prayer permissible at these times and those who consider all (voluntary) prayers disallowed at these times.

Those who consider a reason-based prayer permissible at times of forbiddance, the Shaafi‘i scholars, disallow the Istikhaarah prayer during these times because its reason comes afterwards. They stipulate that the reason should precede the prayer in order to make it allowed at times of forbiddance. Therefore, they disallowed the two Rak‘ahs of Ihraam as well as the Istikhaarah prayer at such times, because the reason for such prayers comes after they are performed.

The Hanbali, Hanafi and Maaliki scholars, on the other hand, disallow the Istikhaarah prayer at such times due to the general forbiddance.  

Allaah Knows best.