Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
The Prophet (peace and blessings be upon him) said, “Actions are judged according to intentions.” (Al-Bukhari)
Thus if a person uses such words simply by way of endearment or as an expression of respect and admiration, they do not have any legal consequences.
Imam Ibn Qudamah states in his reputable work, Al-Mughni,
“If someone says to his wife you are like my mother or sister with the intention of Zhihar, then it would be considered as Zhihar, according to the majority of scholars. If, on the other hand, he uses the words by way of showing respect and appreciation, they would not be considered as such.”
However, it is best to stay away from such expressions, as some scholars have ruled.
Using these words intentionally undoubtedly entails serious consequences as explicitly stated in the following verse:
“As for those of you who [henceforth] separate themselves from their wives by saying, “You are as unlawful to me as my mother”, [let them bear in mind that] they can never be [as] their mothers: none are their mothers save those who gave them birth: and so, behold, they but utter a saying that runs counter to reason, and is [therefore] false. But, behold, God is indeed an absolver of sins, much-forgiving: hence, as for those who would separate themselves from their wives by saying, “You are as unlawful to me as my mother”, and thereafter would go back on what they have said, [their atonement] shall be the freeing of a human being from bondage before the couple may touch one another again: this you are [hereby] exhorted to do – for God is fully aware of all that you do.” (Al-Mujadilah 58:2-3)
Given the seriousness of such utterances, it behooves every conscientious Muslim to heed the order of the Prophet (peace and blessings be upon him) to stay away from that which is doubtful for by doing so we are better enabled to safeguard our faith and honor.
Allah Almighty knows best.