It is narrated from Sayyiduna Nu'man bin Bashir (may Allah be pleased with him) that (he said): I know the time of this prayer (Isha) better than anyone. The Messenger of Allah (peace and blessings be upon him) used to pray it at the time the moon set on the third night.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1748