Is it forbidden for a husband to insert his finger in his wife's vagina?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Inserting a finger into vagina of a wife is not prohibited. However, one should avoid such practice which is disliked by nice-natured people. Unfortunately some people indulge in such silly matters till they develop worst practices. Instead they should always be fond of noble and good manners.
Allah knows best.