2a] The meaning of the hadeeth “ ‘Qul Huwa Allaahu Ahad” is equivalent to one-third of the Qur’aan”

2a]
The Muslim cannot do without the two other issues, which are the
rulings and the promises and warnings. His knowledge cannot be
complete unless he looks at the Book of Allaah as a whole. The one who
stopsat Soorat al-Ikhlaas cannot know the other two matters.
Shaykh al-Islam (Ibn Taymiyah – may Allaah have mercy on him) said:
Rewards are of different types, just as wealth is of different types,
such as food, drink, clothing, houses, money and so on. If a man
possesses one type of wealth, to the value of one thousand dinars,
that does not mean that he can do without the other types. Rather if
he has wealth in the form of food, he also needs clothing and a place
to live, etc. Similarly if it is a type other than money, he still
needs other things. If he has nothing but money, he will need all
kinds of wealth that are usually needed. In al-Faatihah there are the
benefits of praise and du'aa' which people need, and Qul Huwa Allaahu
Ahad cannot replace it in that sense. Although its reward is very
great, he cannot benefit from it unless he also recites the Opening of
the Book (al-Faatihah) in his prayer.Hence if a person recites only
Qul Huwa Allaahu Ahad in his prayer, without al-Faatihah, his prayer
is not valid. Even if he recited the whole of the Qur'aan without
al-Faatihah, his prayer would not be valid, because the al-Faatihah
refers to the basic needs that people cannot do without.
Majmoo' al-Fataawa, 17/131.
And he said:
The people need the commands, prohibitions and stories that are in
the Qur'aan, even though Tawheed is greater than that. Man needs to
know what he is enjoined to do and what he is forbidden to do; he need
to know what is enjoined upon him and the stories and promises and
rewards. These cannot be replaced by anything else, and Tawheed cannot
be replaced by these. The stories cannot take the place of the
commands and prohibitions, and the commands and prohibitions cannot
take the place of the stories. Rather everything that was sent down by
Allaah is beneficial and people need it.
If a person recites Qul Huwa Allaahu Ahad, he earns a reward
equivalentto the reward of one-thirdof the Qur'aan, but that does not
mean that the reward is of the same kind as that earned by reading the
rest of the Qur'aan. Rather he may need the kind of reward that comes
from reading the commands and prohibitions and stories, so Qul Huwa
Allaahu Ahadcannot take the place of all that.
And he said:
The knowledge that is to be gained by reading the rest of the Qur'aan
cannotbe gained by reciting this soorah only. So whoever reads the
whole Qur'aan is better than one who simply recites this soorah three
times in the sense that he earns different kinds of reward, even
though the one who recites Qul Huwa Allaahu Ahad earns a reward
equivalent to that reward,but it is of one type and does not include
all the types that a person needs.This is like a man who hasthree
thousand dinars andanother who has food, clothing, accommodation and
money equivalent to three thousand dinars. The latter has that which
will benefit him in all his affairs, whereas the former needs what the
latter has, even though what he has is equivalent in value. Similarly,
if he has the finest food, equivalent to three thousand dinars in
value, he still needs clothing andaccommodation, and weapons and tools
that will ward off harm from him, and the like, which cannot be done
with foodalone.
Majmoo' al-Fataawa,

And Allaah knows best.