1) Allaah the Most High says:
Those who believe and do not mix their belief with dhulm, they 
are those upon whom is security and they are the rightly guided. [6:82]
From 'Abdullaah ibn Mas'ood, who said: When this verse was revealed, some 
Muslims began to grieve, and said: 'Which of us does not do dhulm 
(oppression) to his soul?' The Messenger of Allaah said: "It is not 
that, verily it is shirk, have you not heard the saying of Luqmaan to his 
son: 
Oh my Son! Do not make shirk with Allaah. Verily, shirk is a great 
dhulm. [31:13] [Bukhari & 
Muslim]
This verse gives glad tidings to the believers, those who single out Allaah 
alone for worship, those who do not mix their belief with shirk. Rather, they 
are far from it, they have complete security from the punishment of Allaah in 
the Hereafter and they are the rightly guided in this world.
2) The Prophet said: "Imaan consists of sixty something branches, the 
highest of them is the saying 'Laa ilaaha illa Allaah; and the lowest of them is 
to remove something harmful from the road." [Muslim]
3) Shaikh 'Abdullaah Khayaat mentions in his book entitled Daleel 
ul-Muslim Fil I'tiqaad wat-Tat-heer. "Because of the human nature of 
man and due to his lack of infallibility, his feet slip and he falls into 
disobedience to Allaah. When he is from the people of tawheed, pure from the 
blemishes of shirk, and he singles out Allaah alone for worship, and he is 
sincere in saying 'Laa ilaaha illa Allaah' it becomes the greatest factor 
for his happiness and an expiation for his sins and an elimination of his evil 
deeds, as has come in the hadeeth of the Messenger of Allaah: " Whoever 
testifies that none has the right to he worshipped except A llaah, alone without 
any partners and that Muhammad is His slave and His messenger and that 'Eesaa is 
a slave of Allaah, and a Word delivered to Maryam and a Spirit from Him, and 
that Paradise is true and the Fire is true, Allaah will enter him into Paradise 
in accordance to his deeds." [Bukhari & Muslim]
Verily, 'Laa ilaaha illa Allaah', the testification that the Muslim 
testifies with, is the foundation necessary for his entrance into Paradise, the 
place of comfort, even if some of his actions had defects and shortcomings. As 
has come in the hadeeth Qudsee, Allaah, the Most High, says: "O Son of 
Aadam, verily If you were to come to Me with the equivalent of the earth full 
ofmistakes and then meet Me without associating partners with Me in anything 
then Iwould give to you its equivalent amount of Mercy.) [Hasan - Tirmidhi & 
ad-Diyaa]
Meaning: If one of you came to Me with an equivalent amount of the earth 
full of sins and disobedience, but you died upon tawheed, then I would 
forgive your sins.
It is reported from the Prophet that he said: "Whoever meets 
Allaah without having associated with Him in anything will enter Paradise 
and whosoever meets Him associating partners with him in anything will 
enter the Fire. " [Muslim]
All these narrations clearly show the excellence of tawheed, for it is that 
by which the happiness of the slave is achieved and the best means for the 
expiation of his sins and the elimination of his 
mistakes."