Musnad Ahmad 1153:1191

Abu Abd al-Rahman narrated to us, Sa'id narrated to us, Ja'far ibn Rabi'ah narrated to me, from 'Arak ibn Malik, from Abu Salamah, from 'Aishah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) prayed the Isha prayer, then he prayed eight rak'ahs standing, and two rak'ahs sitting between the two calls (the Adhan and the Iqamah); he would not abandon them."