Narrated Abu Musa:
The Prophet said, 'The example of a believer who recites the Qur'an is that of a citron (a citrus fruit) which is good in taste and good in smell. And the believer who does not recite the Qur'an is like a date which has a good taste but no smell. And the example of an impious person who recites the Qur'an is that of Ar-Rihana (an aromatic plant) which smells good but is bitter in taste. And the example of an impious person who does not recite the Qur'an is that of a colocynth which is bitter in taste and has no smell."
English reference: Vol. 9, Book 93, Hadith 649
Arabic reference: Book 98, Hadith 7655