Ibn Abbas (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) said: "I have not been commanded to build mosques lavishly." Ibn Abbas (may Allah be pleased with them both) said: You will surely adorn them as the Jews and the Christians did.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4298