Which scriptures deal with rules for widows?
The Dharma Shastras prescribe rules for widows.
Verses from various Dharma Shastras below:
Parāśara (4.29).—‘If, on the death of her husband, a woman remains
firm in her chastity, she obtains heaven, on death, in the manner of
the Religious Students.’
Śukranīti (4.4.57-59).—‘On the death of her husband, the chaste woman
should either accompany him or observe the vows; she should not go to
other houses; she should maintain chastity, control her passions and
give up personal adornment.’
More verses from the Manusmriti:
The good wife, desirous of reaching her husband’s regions, should
never do anything that may be disagreeable to her husband, alive or
dead.—(154).
Well might she macerate her body by means of pure flowers, roots and
fruits; but she should not even mention the name of another man, after
her husband is dead—(155).
Till her death, she should remain patient, self-controlled and
chaste,—seeking that most excellent merit that accrues to women having
a single husband. (156).
Many thousands of unmarried Brāhmaṇa students have gone to heaven,
without having perpetuated their race—(157).
On the death of her husband, the good wife who remains firm, goes to
heaven, even though childless; just like those students—(158).
That woman, however, who from a longing for a child, disregards her
husband, brings disgrace to herself in this world and falls off from
her place in the other world.—(159).
What is born of another is not a ‘child’: nor is one begotten on
another man’s wife; for good women a second husband is nowhere
ordained.—(160).
She, who, having abandoned her own husband who is inerior, has
recourse to another person who is superior, becomes contemptible in
this world and is called a ‘remarried woman.’—(161).
The woman, who, through failure in her duty to her husband, becomes an
object of contempt in the world, comes to be born as a jackal and is
tormented by foul diseases.—(162).
She, who does not fail in her duty to her husband, having her thought,
speech and body well-controlled, reaches her husband’s regions; and is
called ‘good’ by all gentle-men.—(163).
By such conduct, the woman, having her thought, word and body well
controlled, obtains excellent fame in this world, and also her
husband’s region in the other world.—(164).
The twice-born man, knowing the law, should cremate the wife of his
own caste,—who behaves herself in the said manner, and dies before
him,—with the sacred fire and along with the sacrificial
implements.—(165).