Allah, the Exalted and High, says: "وَرَتِّلِ الْقُرْآنَ تَرْتِيلا"
'And recite the Qur'an (aloud) in a (slow and melodious) style (tarteela)' [Surat Al-Muzzammil, Ayah 4]
Ali ibn Abi Taalib (radya Allahu anhu), said in explanation of this Ayah: ‘Tarteel is Tajweed of the letters and knowing where to stop (correctly).’
The Prophet Mohammed (salla Allahu alayhi wasallam) said: “The best of you are those that learn the Qur’an and teach it.” [Bukhari]
On the day of Resurrection a crown of such brilliance will be placed on the heads of the parents of one who learnt the Qur'an and practiced on its laws, that its glitter will outshine the brilliance of the sun which penetrates your houses. What then do you think will be the position of the one who himself learnt the Qur'an and acted in accordance with it? (Ahmad, Abu Dawud).
Whoever recites the Qur'an well and clearly is equal in rank to the Angels who record creation's deeds. These Angels are gracious, honorable and of lofty rank. He who finds difficulty in reciting the Qur'an will obtain a double reward. (Bukhari, Muslim)
Whoever recites the Qur'an, memorizes it, accepts its halal (lawful things) as halal and haram (forbidden things) as haram (i.e. his beliefs in these matters are correct), Allah, the Exalted, will enter him to Paradise and accept his intercession on behalf of ten such persons of his family upon whom entry into Hell has become incumbent (i.e. Allah will forgive them because of his intercession)."
(Ahmad, Tirmidhi, Ibn Majah, Darimi).
Imam Mohammed Ibn Al-Jazari (rahimahullaah), who was a great Qur’an and Hadeeth scholar of the 9th century (after Hijrah), stated in his famous poem that details the rules of Tajweed:
‘And applying Tajweed is an issue of absolute necessity,
Whoever doesn't apply Tajweed to the Quran, then a sinner is he.’
Sufyan Ath-Thawri (rahimahullaah), said: ‘This indicates that it is even greater in virtue than fighting in the way of Allah, unless that becomes an individual obligation.’