Chapter: 73, Verse: 4 - How to recite the Holy Quran:

مُحَمَّدُ بْنُ يَعْقُوبَ: عَنْ عَلِيِّ بْنِ إِبْرَاهِيمَ، عَنْ أَبِيهِ، عَنْ عَلِيِّ بْنِ مَعْبَدٍ، عَنْ وَاصِلِ بْنِ سُلَيْمَانَ، عَنْ عَبْدِ اللَّهِ بْنِ سُلَيْمَانَ، قَالَ: سَأَلْتُ أَبَا عَبْدِ اللَّهِ (عَلَيْهِ السَّلَامُ)، عَنْ قَوْلِ اللَّهِ عَزَّ وَ جَلَّ: وَ رَتِّلِ الْقُرْآنَ تَرْتِيلًا، قَالَ:

«قَالَ أَمِيرُ الْمُؤْمِنِينَ (صَلَوَاتُ اللَّهِ عَلَيْهِ): بَيِّنْهُ تِبْيَاناً وَ لَا تَهُذَّهُ هَذَّ الشِّعْرِ، وَ لَا تَنْثُرْهُ نَثْرَ الرَّمْلِ، وَ لَكِنْ أَفْزِعُوا قُلُوبَكُمُ الْقَاسِيَةَ، وَ لَا يَكُنْ هَمُّ أَحَدِكُمْ آخِرَ السُّورَةِ»

The narrator says, I asked Abu Abd Allah Imam Sadiq (عليه السلام) about the saying of Allah (عز وجل):

"And recite the Qur'an with measured recitation (tartil). (73:4)"

The Imam said, "Amir al-Mominin (عليه السلام) said:

Make it clear and distinct.

And do not rush through it as if reciting poetry [i.e. don’t recite it quickly/fast].

Don't scatter it like scattered sand [i.e. don’t recite without connection between words with long pauses].

But rather, put fear into your hardened hearts with it.

And let not the concern of any of you be the end of the surah [i.e. focus should not be to just complete the surah]

[Source: al-Kafi, Vol. 2, Book. 3, Ch. 9, Hadith. 1]