Musnad Ahmad 1153:951

'Abd al-Wahhab narrated to us, he said: Hisham informed us, from Yahya, from Abu Salamah, he said: 'Aishah narrated to me, "That the Prophet, peace and blessings of Allah be upon him, used to pray two rak'ahs between the call (adhan) and the iqamah of the morning prayer."