Praise be to Allah.
Masturbation between husband and wife is not haram, rather, it is permissible. This is because it is a part of the enjoyment which Allah has allowed. Allah says (interpretation of the meaning):
"Those who guard their chastity (.e., [private parts, from illegal sexual acts) Except from their wives or (the captives and slaves) that their right hands possess, - for them, they are free from blame." [al-Muminun 23:5-6]
And Allah knows best.