Ibn Majah

1؎: SubhanAllah! How correct and true is your noble statement, and indeed it is a fact that ever since Muslims began to exaggerate in the adornment and beautification of mosques without necessity, their deeds have deteriorated, the teaching of religious knowledge has decreased, people's interest in religious schools has diminished, people have become negligent regarding the educational expenses of religious students, ignorance is rampant, innovations and superstitions and polytheistic practices have become widespread, yet the embellishment of mosques continues to increase day by day. The greatest adornment and beautification of mosques is that the call to prayer (adhan) is given at the earliest time, the imam is pious and knowledgeable, the prayer is performed in accordance with the Sunnah, with its conditions and etiquettes, lighting is provided as needed so that worshippers are not troubled by darkness, and any surplus funds beyond this should be spent on the advancement of religious knowledge: religious books should be published, religious preachers should be given salaries so that they may teach Muslims about the religion and invite disbelievers and polytheists to Islam, the needs of students should be attended to, teachers should be appointed, schools should be established. If, despite knowing this, someone does not spend his wealth on these matters and instead wastes it on the decoration, embellishment, lighting, and flooring of the mosque, then he will be sinful, and in the Hereafter he will be questioned about it, because he adorned the exterior and ruined the interior.