I am the one who is best informed of the time of this prayer, i.e. the night prayer. The Messenger of Allah ﷺ used to offer it at the hour when the moon went down on its third night. (Using translation from Abū Dāʾūd 419)  

إِنِّي وَاللهِ لَأَعْلَمُ النَّاسِ بِوَقْتِ هَذِهِ الصَّلَاةِ صَلَاةِ الْعِشَاءِ الْآخِرَةِ كَانَ رَسُولُ اللهِ ﷺ «يُصَلِّيهَا لِسُقُوطِ الْقَمَرِ لِثَالِثَةٍ»