Mark 3:30
Because they said, He hath an unclean spirit.
Cross-reference
Mark 3:22
Historical context
Mark 3:22 provides the accusation that Jesus has Beelzebul and casts out demons by the prince of demons, which leads to the statement in 3:30.
John 7:20
Parallel
John 7:20 has the crowd say 'You have a demon!' — the same accusation of demon possession as in Mark 3:30.
John 10:20
Parallel
In John 10:20, many also say Jesus has a demon and is insane — a similar accusation of demon possession in a different context.