Moulana Muhammad Ilyaas (رحمه الله) once mentioned:
When Allah Most High blesses a person to carry out righteous actions and pious works, then regardless of how excellent, virtuous and great these good deeds may be, upon completion one should make istighfaar which is to say, “Astaghfirullah” meaning “I ask Allah’s forgiveness.” In essence, every good action should be concluded with istighfaar.
In doing so, we should think to ourselves that we have definitely fallen short and failed to fulfil this action as it ought to have been fulfilled, and it is on account of our deficiencies and weaknesses that we are making istighfaar. Upon the completion of salaah, The Messenger of Allah (ﷺ) would also make istighfaar.
Therefore, based on the same principle, the effort of da’wah and tableegh (inviting people towards Deen) should also be concluded with istighfaar. We should make istighfaar and think to ourselves that definitely we have failed to carry out this effort in the correct manner that it was supposed to be carried out. Furthermore, due to us being engaged in one type of Deeni effort, we are unable to fulfil the other Deeni efforts. In compensation for this, upon completion of any good action, we should make istighfaar. (Malfoozaat Hazrat Moulana Muhammad Ilyaas -رحمه الله – pg. 31)