Praise be to Allah.
We do not know of any saheeh evidence which refers to the reward mentioned, but al-Tabaraani narrated from Ibn Mas’ood that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah has decreed jealousy for women and jihad for men, so whoever bears that with patience out of faith and in the hope of reward will have a reward like that of a martyr.” This hadeeth was classed as da’eef (weak) by al-Albaani in Da’eef al-Jaami’ al-Sagheer, no. 1626.
A woman’s being patient in obeying her husband is one of the means of entering Paradise, as it says in the hadeeth narrated by Ibn Hibbaan: “If a woman offers her five daily prayers and fasts her month (i.e., Ramadaan) and guards her chastity and obeys her husband, it will be said to her: ‘Enter Paradise from whichever of the gates of Paradise you wish.’” This hadeeth was classed as saheeh by al-Albaani in Saheeh al-Jaami’ al-Sagheer, no. 660.
Her patience in putting up with her husband’s marrying a second wife will bring a special reward over and above that, for several reasons:
1 – Her husband’s marrying another wife is regarded as a test and trial for her, and if she bears that with patience she will have the reward for being patient in the face of a trial, as Allaah says (interpretation of the meaning):
“Only those who are patient shall receive their reward in
full, without reckoning”
According to the hadeeth: “No fatigue, nor disease, nor sorrow, nor sadness, nor hurt, nor distress befalls a Muslim, not even a thorn that pricks him, but Allaah will expiate his sins thereby.”
Narrated by al-Bukhaari, 5642; Muslim, 2573, from the hadeeth of Abu Sa’eed and Abu Hurayrah.
Al-Tirmidhi (2399) narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “Trials will continue to befall the believer, man or woman, concerning himself, his child and his wealth, until he meets Allaah with no sin on him.” Classed as saheeh by al-Albaani in Saheeh al-Jaami’, no. 5815.
2 – If a woman accepts that and treats her husband and the other wife well, she will have the reward of al-muhsineen. Allaah says (interpretation of the meaning):
“Verily, he who fears Allaah with obedience to Him (by
abstaining from sins and evil deeds, and by performing righteous good
deeds), and is patient, then surely, Allaah makes not the reward of the
Muhsinoon (good‑doers) to be lost”
“Is there any reward for good other than good?”
“And verily, Allaah is with the Muhsinoon (good-doers)”
3 – If she feels angry because of that, but she controls her anger – and controlling one's tongue is part of controlling one's anger – Allaah says (interpretation of the meaning):
“…who repress anger, and who pardon men; verily, Allaah loves Al-Muhsinoon (the good‑doers)”
[Aal ‘Imraan 3:134]
This reward is in addition to the woman’s reward for obeying her husband under ordinary circumstances.
A wise woman should accept whatever Allaah decrees for her, and she should realize that her husband’s marrying another woman is something permissible, so she should not object to it. It may be that this (second) marriage will make him more chaste and prevent him from doing something haraam.
It is very unfortunate that some women object less to their husbands doing haraam things than to their marrying another woman in a permissible manner. This is a sign of their lack of reason and religious commitment.
Women should follow the good example of the wives of the Prophet (peace and blessings of Allaah be upon him) and his companions, who were patient and sought reward even though many of them felt jealous. If your husband goes ahead and takes a second wife then you have to be patient and content and treat him well so that you can attain the reward of the patient and the doers of good.
Note that this life is the life of trials and tests, and how quickly it ends. So congratulations to the one who is patient in obeying Allaah in this life until he attains eternal delight in the gardens of Paradise.
And Allaah knows best.