Hadith: We used to set aside for the Prophet (may Allah's peace and blessings be upon him) his share of milk, and he would come at night and offer greetings in a tone that did not disturb those who were asleep but was heard by those who were awake

عن المِقْدَادُ رضي الله عنه في حديثه الطويل: كنا نَرَفَعُ للنبي صلى الله عليه وسلم نَصِيبَهُ من اللَّبَنِ ، فَيَجِيءُ من الليل، فَيُسَلِّمُ تسليما لا يُوقِظُ نائما، ويُسْمِعُ اليَقْظَانَ، فجاء النبي صلى الله عليه وسلم فَسَلَّمَ كما كان يُسَلِّمُ.
[صحيح] - [رواه مسلم]

Al-Miqdād (may Allah be pleased with him) reported: We used to set aside for the Prophet (may Allah's peace and blessings be upon him) his share of milk, and he would come at night and offer greetings in a tone that did not disturb those who were asleep but was heard by those who were awake. So (one night), the Prophet (may Allah's peace and blessings be upon him) came and gave greetings as is he used to greet.
Sahih/Authentic. - [Muslim]

Explanation

Al-Miqdād and those who were with him used to milk their sheep and drink their portion of milk, then they would leave the portion of the Prophet (may Allah's peace and blessings be upon him) until he came to drink it. So, when he would come at night while they were sleeping, he would give greetings (Salām) in a moderate tone that would not wake the sleeping person up, whereas it would be heard by the ones who were awake.