He said to his wife: “You are to me like my mother”

Firstly: 

This phrase
is not regarded as a talaaq divorce, rather it is a kind of zihaar [a
jaahili form of divorce in which the husband says to his wife, “You are to
me as my mother’s back”] which is not explicitly zihaar, rather it may be
understood as zihaar or as something else. 

Secondly: 

The ruling
on this phrase depends on the intention of the speaker (the husband) and the
context. 

As for the
intention, if the husband intended by saying that that she was forbidden to
him like his mother, then it is zihaar. 

Or he may
have meant that she is like his mother in honour and love etc, in which case
it is not zihaar, and does not mean anything.  

As for
context, if the context of words and events that came before this indicate
that the husband intended zihaar, then it is zihaar. Examples of such
contexts are: if the husband was having an argument with his wife and during
that argument he said, “You are to me like my mother.” This context suggests
that he intended zihaar so it is zihaar, 

Shaykh
al-Islam Ibn Taymiyah was asked in Majmoo’ al-Fataawa (34/5) about a
man who said to his wife: “You are to me like my mother or my sister.” He
replied: If what he meant was you are to me like my mother or my sister in
terms of honour, then he does not have to do anything, but if what he meant
was likening her to his mother or sister in terms of marriage, then this is
zihaar and he has to do the same as the one who divorces his wife by zihaar:
if he chooses to keep her, he should not approach her until he has offered
the expiation for zihaar.  End quote. 

He also said
(34/7): 

If he meant
when he said, “you are like my mother to me” that he would refrain from
physical relations with her which are forbidden in the case of the mother,
then this is zihaar and he is obliged to do what the man who divorces his
wife by zihaar is obliged to do. It is not permissible for him to have
intercourse with her until he has offered the expiation for zihaar, namely
freeing a slave. If that is not possible then he must fast for two
consecutive months. If he cannot do that then he must feed sixty poor
persons. End quote. 

Ibn Qudaamah
said in al-Mughni (11/60): 

If he says:
You are to me like my mother, intending zihaar thereby, then it is zihaar,
according to the majority of scholars, including Abu Haneefah and his two
companions, al-Shaafa’i and Ishaaq. If he intended thereby to show honour
and respect, or to say that she is like her in age or in some other way,
then this is not zihaar. What matters is what he says about his intention.
End quote. 

i.e., the
husband is the one who has to clarify what he intended. 

It says in
Fataawa al-Lajnah al-Daa’imah (20/274): 

If the
husband says to his wife: I am your brother or You are my sister or You are
my mother or like my mother, or You are to me like my mother or my sister,
if he means thereby that she is like them in honour, family ties or respect,
or he did not have any particular intention, or there is no context to
suggest that he intended zihaar, then this does not result in zihaar and he
does not have to do anything. But if he intended zihaar with these or
similar words, or there is a context that indicates zihaar, such as these
words being uttered in anger or as a threat to her, then it is zihaar. This
is haraam and he must repent, and also offer expiation before he touches
her. The expiation is to free a slave; if that is not possible then he must
fast for two consecutive months. If he cannot do that then he must feed
sixty poor persons. End quote. 

To sum up: 

If the
husband intended zihaar by these words, or if there is a context that points
to that, then it is zihaar. If it is other than that, then it is not zihaar
and he does not have to do anything. 

Thirdly: 

Zihaar is
haraam, and Allaah has described it as an ill word and a lie, and He says
(interpretation of the meaning): 

“Those
among you who make their wives unlawful to them by Zihaar (i.e., by saying
to them “You are like my mother’s back,”) they cannot be their mothers. None
can be their mothers except those who gave them birth. And verily, they
utter an ill word and a lie”

[al-Mujaadilah 58:2]

The one who
speaks the words of zihaar to his wife has to repent to Allaah, may He be
exalted.

Fourthly: 

The husband
who divorces his wife by zihaar has to offer expiation if he wants to keep
her and not divorce her and it is not permissible for him to have
intercourse with her until he has offered expiation. The expiation is to
free a slave. If that is not possible then he must fast for two consecutive
months. If he cannot do that then he must feed sixty poor persons. The
evidence for that is the verse in which Allaah says (interpretation of the
meaning): 

“And
those who make unlawful to them (their wives) by Zihaar and wish to free
themselves from what they uttered, (the penalty) in that case is the freeing
of a slave before they touch each other. That is an admonition to you (so
that you may not repeat such an ill thing). And Allaah is All‑Aware of what
you do.

4. And he
who finds not (the money for freeing a slave) must fast two successive
months before they both touch each other. And he who is unable to do so,
should feed sixty Masaakeen (poor). That is in order that you may have
perfect faith in Allaah and His Messenger. These are the limits set by
Allaah. And for disbelievers, there is a painful torment”

[al-Mujaadilah 58:3-4].