Surah Al-Muzzammil 73:4

أَوْ زِدْ عَلَيْهِ وَرَتِّلِ ٱلْقُرْءَانَ تَرْتِيلًا

Or add to it, and recite the Qur’ān with measured recitation.

Translation: Saheeh International

Tafsir

or add to it up to two thirds aw implies free choice and recite the Qur’ān recite it carefully in a measured tone.

Excerpt from Tafsir al-Jalalayn, commentary on 73:4, via Quran.com. The reader has the full commentary and 9 more tafsirs.