Shaikh Muḥammad Naṣir Al-Dīn Al-Albānī, may Allāh have mercy on him, said:
The beautification of mosques is among the innovations that have infiltrated Islam, since he [ﷺ] said: “I have not been commanded to be extravagant in building mosques.” The one who narrated the ḥadīth is ʿAbd Allāh ibn ʿAbbās, and he said, after he narrated the ḥadīth: “You will most definitely beautify them just as the Jews and Christians have beautified [them].”
So it’s not allowed to beautify mosques from this angle, first of all; then, second, from the angle that there’s a waste of money in [this], money that belongs to the Muslims; after that, [it’s not allowed] because of what’s in [this] of busying [people]—busying the thoughts of those praying in these mosques with the decorations that have been painted or plastered on their walls.
Due to [this], it has come [to us] in Ṣaḥīḥ Al-Bukhārī that ʿUmar ibn Al-Khaṭṭāb, when he [was renovating] the structure of the Prophet’s Mosque, told the builders: “Keep people covered [and protected] from the heat and cold and neither give things a red color nor give things a yellow color.”
Silsilah Al-Hudá wa-l-Nūr, 169, 00:44:36.