Shama-il Tirmidhi Chapter 043, Hadith Number 004 (300).

'Abdullah bin Abi Qays Radiyallahu 'Anhu reports: "I inquired from 'Aayeshah Radiyallahu 'Anha about the recital of Rasulullah Sallallahu 'Alayhi Wasallam, did he 'recite softly or audibly? She replied: 'He recited in both ways'. I said: 'Alhamdulillaah, (Praise be to, Allah), who has accorded us easiness in the matter"'. (According to the situation whatever was proper, reciting softly or audibly was adopted).

Commentary
This question, as has been mentioned in the jaami' Tirmidhi, is regarding the recital at the time of tahajjud. Sayyiditina 'Aayeshah Radiyallahu 'Anha replying that he recited both softly, and audibly, may also mean that he recited both tones in the same night.

This is also correct that in one night he recited the entire salaah audibly, and on another night he recited the entire salaah softly. Both are correct, and there is no harm in reciting in both manners. At times it is better to recite audibly, especially where it is intended to persuade others, or it will create strength to recite. Where there is a possibility that one will be troubled, or where it is doubted that this will create pride, it is better not to recite audibly as it is important to consider those present. When sometimes both are of the same nature, then it is advisable to recite in a slightly audible tone. It was the practice of Sayyidina Abubakr Radiyallahu 'Anhu to recite softly in the tahajjud. Sayyidina 'Umar Radiyallahu 'Anhu used to recite audibly. Once Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam passed both at night and witnessed the practices of both. When they attended the noble gathering of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam in the morning Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam discussed this. Sayyidina Rasulullah Sallallahu 'Alayhi Wasallarn said to Sayyidina Abubakr Radiyallahu 'Anhu. "When I passed you, you were reciting softly". He replied: "That great being with whom I was conversing was listening". Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam then said to Sayyidina 'Umar Radiyallahu 'Anhu: "When I passed you, you were reciting loudly". Sayyidina 'Umar Radiyallahu 'Anhu replied: "The intention was to awaken those who were carelessly sleeping, and to keep away the shaytaan". Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam approved the explanations of both, and said to Sayyidina Abubakr Radiyallahu 'Anhu "Raise your voice a little" and to Sayyidina 'Umar Radiyallahu 'Anhu: "Lower your voice a little".