What is the ruling on the mihrab? And is it permissible for the imam to lead the congregational prayer from within it?

Some of the Salaf disliked the adoption of the mihrab (prayer niche) and praying in it, due to its absence during the time of the Prophet (peace and blessings be upon him) and the Rightly Guided Caliphs, considering it an innovation (bid'ah). However, the practice of the Ummah settled on adopting it. The first to introduce it was Umar ibn Abd al-Aziz when he built the Prophet's Mosque in the year 91 AH. People have continued to act upon this without objection, and the mihrab benefits strangers in determining the direction of the qiblah. There is no harm in the Imam praying within it and the congregation behind him.