Enhancing the already present splendor of the Taj Mahal is a building that stands on the western side of it, a Mosque made up of red sandstone. The mosque and a mirror image of the mosque, a guest house that stands on the opposite side of it, together provide a perfect symmetrical balance to the architecture whole of Taj Mahal.
The interiors host an elegantly designed floor that is made up of a material that appears to be velvet red in shade and is in the shape of clearly defined prayer mats, 569 prayer mats in total. The interiors of the mosque are inscribed with delicate calligraphy citing the name Allah and quotations from scriptures (taken from Sura 91, The Sun, taken from the Quran). However, the main feature of the mosque that distinguishes it from the opposite structure of the guest house is the presence of Mihrab and Minbar.
Additionally, there lies a small stone enclosed space of 19 ft by 6.5 ft, which had served as a temporary grave where the remains of Mumtaz Mahal were kept for some time when they were first brought to Agra, until they finally found an eternal place of rest inside the beautiful mausoleum built in her memory.
Also, the exteriors of the mosque, crypt and cenotaphs carry pietra dura decoration of a fabulous unexcelled elegance. The name of Allah and verses from the Holy Qur’an has been used copiously all over the mosque. And the pool in front of the mosque functions as the place for ablution before the prayer.
(Paraphrased from https://www.tajmahal.org.uk/mosque.html)