Musnad Ahmad 5:329

Yahya ibn Sa'id al-Umawi narrated to us, Ibn Abi Layla narrated to us, from Ibn al-Asbahani, from a grandmother of his who was a concubine of Ali, she said: Ali said: "I was a man who slept a lot. When I prayed the Maghrib prayer while wearing my clothes, I would sleep." Then Yahya ibn Sa'id said: "So I would sleep before the Isha prayer." He said: "So I asked the Messenger of Allah, peace and blessings of Allah be upon him, about that, and he granted me a concession."