Musnad Ahmad 1153:2357

Al-Walid ibn al-Qasim ibn al-Walid narrated to us, he said: Zakariyya narrated to us, he said: Khalid ibn Salamah narrated to us, from al-Bahiy, from Urwah ibn al-Zubayr, from Aisha, she said: The Messenger of Allah, peace and blessings of Allah be upon him, used to remember Allah in all of his states.