SIGNIFICANCE OF SAYING "LA ILAHA ILLALLAH"

Abu Sa'eed AI-Khudri narrated that Allah's Messenger (pbuh) said,
"Musa (Moses) said: 'O my Lord, teach me something by which I can remember You and supplicate to You.' 
Allah answered: 'Say, O Musa, La ilaha illallah (none has the right to be worshipped but Allah).' 
Musa said: 'O my Lord, all your slaves say this.'
Allah said: 'O Musa, if the seven heavens and all of their inhabitants besides Me, and the seven earths were in a pan (of a scale), and La ilaha illallah was in (another) pan, then La ilaha illallah would outweigh them."

Ibn Hibbaan and AI-Haakim recorded this, and he (AIHaakim) graded it authentic (Saheeh).
-----

Tirmithi recorded a Hadith that he graded good (Hasan), from Anas (who said):
"I heard Allah's Messenger saying,
"Allah the Most High said, "O son of Aadam! If you came to Me with sins filling the earth, then you met Me without associating anything with Me, I would come to you with its (the earth's) fill of forgiveness.'"
-----

Narrated Anas:
The Prophet said, "Whoever said "(la ilaha ilallah) None has the right to be worshipped but Allah and has in his heart good (faith) equal to the weight of a barley grain will be taken out of Hell."

And whoever said: "(la ilaha ilallah) None has the right to be worshipped but Allah and has in his heart good (faith) equal to the weight of a wheat grain will be taken out of Hell."

And whoever said, "(la ilaha ilallah) None has the right to be worshipped but Allah and has in his heart good (faith) equal to the weight of an atom will be taken out of Hell."
(Sahih Bukhari - Volume 1, Book 2, Number 42)
-----

“So know (O Muhammad SAW) that La ilaha ill-Allah (none has the right to be worshipped but Allah), and ask forgiveness for your sin, and also for (the sin of) believing men and believing women.” Quran (47:19)

It was once said to Al Hasan Al-Basri: "Some people say that those who proclaim La iláha illalláh shall enter Paradise." He said, "Yes. For those who proclaim this word and fulfill its implications and requirements, then saying La iláha illalláh shall enter them into Paradise."
--------------------------------------