Musnad Ahmad 652:50

Yazid narrated to us, Shu'bah informed us, from Abu Bishr, from Bashir ibn Thabit, from Habib ibn Salim, from an-Nu'man ibn Bashir, who said: "I am indeed among the most knowledgeable of people, or one of the most knowledgeable of people, regarding the time of the Messenger of Allah's (peace and blessings of Allah be upon him) Isha prayer. He used to pray it at the interval of the setting of the moon on the third or fourth night."