Musnad Ahmad 1153:1544

'Abd al-Rahman narrated to us, he said: Sufyan narrated to us, from Mansur, from Ibrahim, from 'Alqama, he said: I asked 'Aisha, "Did the Messenger of Allah, peace and blessings of Allah be upon him, single out any of the days [for specific worship]?" She said: "No. His work was constant. And which of you can endure what the Prophet, peace and blessings of Allah be upon him, could endure?"