Is it permissible to designate a daily portion of seeking forgiveness (Istighfar) after the obligatory prayers and before rising from the prayer mat, and similarly during the legally sanctioned excuse (menstruation) after the Adhan and between the Adhan and the Iqamah?

Praise be to Allah. There is no harm for a Muslim to seek Allah's forgiveness a number of times daily. It is authentically reported that the Prophet (peace be upon him) used to seek Allah's forgiveness a hundred times a day. However, allocating a specific time for seeking forgiveness has not been authentically reported from him (peace be upon him), except for three times after concluding the prayer. Specifying a particular time for seeking forgiveness is considered an additive innovation (bid'ah idafiyyah), but if a Muslim does it occasionally, there is no harm in that. Authentic hadiths have been reported confirming that the Prophet (peace be upon him) used to seek Allah's forgiveness between seventy and a hundred times a day.