Is it permissible to make certain invocations (adhkar) and supplications (du'a), whose virtue has been mentioned absolutely or restricted to a specific time—such as the supplication of Sayyid al-Istighfar, reciting Surat al-Ikhlas three times, and the last verses of Surat al-Baqarah—a fixed litany after every prayer?

There is no harm in setting a specific time for general remembrances (dhikr mutlaq) that suits a person's circumstances, such as reciting Sayyid al-Istighfar (the master of seeking forgiveness) after Fajr or Asr prayer. It is: "O Allah, You are my Lord. There is no god but You. You created me, and I am Your slave, and I am faithful to Your covenant and Your promise as much as I can. I seek refuge with You from the evil of what I have done. I acknowledge Your favor upon me, and I acknowledge my sin. So forgive me, for none forgives sins except You."