Please see:
http://www.qurancomplex.com/qfatwa/display.asp?f=5&l=eng&ps=subFtwa
quote:
Sheikh Al-Islam Taqiud-Din Abul-'Abbas Ahmad bin Taymiyah praised Allah and considered this issue among the most complicated issues regarding which so many scholars gave their opinions. Those scholars include jurists, scholars specialized in the field of the sciences of the Holy Qur'an, scholars of Hadith, commentators of the Holy Qur'an, scholars of scholastic theology, the scholars specialized in the field of the strange Hadiths and other scholars.
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When you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally. Certainly, Allâh is Ever a Careful Account Taker of all things. (Translation of Surat An-Nisa, Ayah 86)