Matthew 12:31 is a parallel account of the same saying about blasphemy against the Holy Spirit.

Matthew 12:32 adds the detail about speaking against the Son of Man being forgivable, contrasting with the unforgivable sin.

Luke 12:10 is another parallel account of the unforgivable sin, emphasizing the distinction between speaking against Jesus and against the Spirit.

Leviticus 24:16 prescribes death for blaspheming God's name — the OT background for the seriousness of blasphemy Jesus addresses here.