The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fada'il)

Muslim :: Book 30 : Hadith 5821

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.