Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates: "The Messenger of Allah (peace and blessings be upon him) used to delay the Isha prayer until a third of the night had passed. For this reason it is called 'Atamah', because he (peace and blessings be upon him) used to pray it late (when darkness had fallen)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12834