[2:234] And those of you who pass away, leaving wives, they shall wait by themselves for four months and ten. When they have reached their term, then you would not be at fault regarding what they may do with themselves, honorably; God is aware of what you do.
Because the right of the dead is more important since his separation is not by choice, the period [for the widow] to complete used to be longer. In the beginning of Islam, the waiting period after death was one year. However it reverted to four months and ten days to verify that no sperm of the husband is present in the womb. When the waiting period comes to an end, she is permitted to marry another husband. No one's fidelity lasts to the end of life, as it is said:
Just as faces
decay in the ground
so does the grief
of [women].