Sahih Muslim | Book 30 | Hadith Number 5821

Narrated by Amir b. Sa'd
Amir b. Sa'd reported on the authority of his father that Allah's Messenger (may peace be upon him) said: The greatest sinner amongst the Muslims is one who asked about a thing (from Allah's Apostle) which had not been forbidden for the Muslims and it was forbidden for them because of his persistently asking about it.