Lying is one of the greatest sins
A tradition from the Holy Prophet (S) declares,
“Beware I inform you regarding the greatest of the mortal sins: Associating anything with Allah, disobeying parents and lying!”
(Wasa’il ul-Shia)
A similar tradition has been quoted from Imam Hasan al-Askari (a.s.) also,
“All the evils have been locked in a room and its key is lying.”
(Mustadrak ul-Wasa’il)