Musannaf Ibn Abi Shayba 3:957

Hushaym narrated to us, from Abu Bishr, from Habib ibn Salim, from al-Nu’man ibn Bashir, who said: I am among the most knowledgeable of people, or like the most knowledgeable of people, regarding the time of the Messenger of Allah’s (ﷺ) Isha prayer. He used to pray it after the setting of the moon on the third (or second) night of the beginning of the month.