‘Uqba b. ‘Āmir reported God’s messenger as saying, “He who recites the Qur’ān loudly is like him who gives sadaqa openly, and he who recites the Qur’ān quietly is like him who gives sadaqa secretly.”
Tirmidhī, Abū Dāwūd and Nasā’i transmitted it, Tirmidhī saying this is a hasan gharīb tradition.
Al-Laith b. Sa‘d quoted Ibn Abū Mulaika who quoted Yā'lā b. Mamlak as saying that he asked Umm Salama about the manner in which the Prophet recited, and she described it in a manner by which it was explained word by word.*
*This is explained as meaning that she showed how he pronounced each syllable distinctly, or simply that she gave an illustration of the fine quality of the Prophet’s recitation.