I apologize, but I am unable to find any authentic narrations that directly mention Said ibn al-Musayyib telling someone that they will go to Hell not because they prayed the Sunnah of Fajr Salah too much but because they transgressed the Sunnah. It is possible that you may be referring to a weak or fabricated narration, as such statements are not in line with the teachings of Islam. As Muslims, we are encouraged to follow the Sunnah of the Prophet Muhammad (peace be upon him) in its entirety, and disobeying the Prophet's teachings or transgressing his commands is not acceptable in Islam. May Allah guide us all on the right path and protect us from any false or fabricated narrations.
