It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that when he stood (for tahajjud) at night, he would sometimes raise his voice and sometimes keep it low, and he used to say that the Messenger of Allah (peace and blessings be upon him) also did the same. The chain of this hadith is sahih (authentic), but they did not record it.

Abdullah bin Abi Qays narrates that he asked Sayyidah Aishah (may Allah be pleased with her) how the Messenger of Allah (peace and blessings be upon him) recited at night: did he recite aloud or silently? She said: He (peace and blessings be upon him) did both; sometimes he recited aloud and sometimes silently. So I said: All praise is for Allah, who has made latitude in this matter. This hadith is sahih (authentic) on the condition of Imam Muslim, and it is a supporting witness (shahid) for the previous hadith of Abu Hurayrah (may Allah be pleased with him).

It is narrated from Sayyiduna Abu Qatadah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) passed by Sayyiduna Abu Bakr (may Allah be pleased with him) while he was praying in a low voice, and he (peace and blessings be upon him) passed by Sayyiduna Umar (may Allah be pleased with him) while he was praying in a loud voice. When both of them gathered with the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) said to Abu Bakr: "O Abu Bakr! I passed by you while you were praying in a low voice?" He said: (O Messenger of Allah!) I have made the One I was conversing with hear me. Then he (peace and blessings be upon him) said: "O Umar! I passed by you while you were raising your voice!" He said: O Messenger of Allah! I hope for reward through it, that I may wake the drowsy one. The narrator says: So he (peace and blessings be upon him) said to Abu Bakr: "Raise your voice somewhat," and said to Umar: "Lower your voice somewhat." This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs did not record it.

Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) observed i'tikaf in the mosque, and he (peace and blessings be upon him) heard the people reciting aloud while he (peace and blessings be upon him) was in his tent. So he (peace and blessings be upon him) drew back the curtains and said: "Indeed, each one of you is conversing intimately with his Lord, so let none of you harm another, and let none of you raise his voice over another in the recitation of the prayer." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

It is narrated from Sayyiduna Abu al-Darda (may Allah be pleased with him), who traces it back to the Prophet (peace and blessings be upon him), that he (peace and blessings be upon him) said: "Whoever comes to his bed intending to get up at night and pray, but his eyes overcome him until morning comes, what he intended will be written for him, and his sleep will be a charity to him from his Lord." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it; and in my view, they regarded it as defective because it is also narrated from Za'idah as mawquf (a statement of the Companion):

The previous hadith of Sayyiduna Abu al-Darda (may Allah be pleased with him) is narrated as his own statement (mawquf). And this does not weaken the hadith, because al-Husayn bin Ali al-Ju'fi is earlier, a better memoriser, and more expert in the hadith of Za'idah than the other narrators.

Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Do not single out the day of Friday among the days for fasting, nor single out the night of Friday among the nights for standing in prayer." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it!