If Surah Ikhlas is 1/3 of Quran Then Why Read The Whole Quran?
Firstly: there follow some of the ahaadeeth narrated from the Prophet (peace and blessings of Allah be upon him) which state that Soorat al-Ikhlaas (Qul Huwa Allahu Ahad) is equivalent to one-third of the Quran.
Al-Bukhaari (6643) narrated from Abu Saeed that a man heard another man reciting Qul Huwa Allahu Ahad and repeating it. The next morning he came to the Messenger of Allah (peace and blessings of Allah be upon him) and told him about that. The man thought that it was too little, but the Messenger of Allah (peace and blessings of Allah be upon him) said: “By the One in Whose hand is my soul, it is equivalent to one-third of the Quran.”
Muslim (811) narrated from Abul-Dardaa that the Prophet (peace and blessings of Allah be upon him) said: “Is any one of you unable to recite one-third of the Quran in one night?”
They said, “How could anyone read one-third of the Quran?”
He said, “Qul Huwa Allahu Ahad is equivalent to one-third of the Quran.”
Muslim (812) narrated that Abu Hurayrah said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Gather together, for I will recite to you one-third of the Quran.”
So those who could gather together gathered there, then the Prophet of Allah (peace and blessings of Allah be upon him) came out and recited Qul Huwa Allahu Ahad, then he went in.
They said to one another, Perhaps there has been some news from heaven on account of which he has gone inside (the house).
Then the Prophet of Allah (peace and blessings of Allah be upon him) came out and said, “I told you that I was going to recite to you one-third of the Quran. Verily it is equivalent to one-third of the Quran.”
Secondly: The bounty of Allah is immense, and Allah has bestowed His bounty upon this ummah and has made up for its short life span by giving it more reward for simple deeds. It is strange that with some people, instead of this motivating them to do more good, this makes them apathetic and lazy in doing acts of worship, or they feel that this bounty and reward is strange and farfetched.
With regard to the meaning of the hadeeth:
There is a difference between jaza (reward) and ijza (what is sufficient). What is making the brother confused is that he does not see the difference between them.
Jaza means the reward which Allah gives for obeying Him.
Ijza means what is sufficient and takes the place of something else.
Reciting Qul Huwa Allahu Ahad brings a reward equivalent to reciting one-third of the Quran, but it does not take the place of reading one-third of the Quran.
If a person vows – for example – to read one-third of the Quran, it is not sufficient for him to read Qul Huwa Allahu Ahad, because it is equivalent to one-third of the Quran in reward, but not in terms of being sufficient or taking the place of reading one-third of the Quran.
The same may be said of reciting it three times. If a person recites it three times in his prayer, that does not mean that he does not have to recite al-Faatihah, even though he will be given the reward of reciting the whole Quran.
A similar example is the reward given by the Lawgiver to one who offers a single prayer in the Sanctuary of Makkah, and that he will have the reward of one hundred thousand prayers. Does anyone take this divine bounty to means that he does not have to pray for decades because he offered a single prayer in the Haram that is equivalent to one hundred thousand prayers?
Rather this has to do with reward; as for what is sufficient, that is another matter altogether.
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.
And he said:
The knowledge that is to be gained by reading the rest of the Quran cannot be gained by reciting this soorah only. So whoever reads the whole Quran 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 Allahu 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 has three thousand dinars and another 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 and accommodation, and weapons and tools that will ward off harm from him, and the like, which cannot be done with food alone.
Majmoo al-Fataawa, 17/137-139