Musnad Ahmad 5:578

Abdullah narrated to us, Abu Bakr ibn Abi Shaybah narrated to us, Abu Usamah narrated to us, from Abdullah ibn Muhammad ibn Umar ibn Ali, from his father, from his grandfather, that Ali used to travel until, when the sun set and it became dark, he would descend and pray Maghrib, then he would pray Isha immediately after it. Then he would say: "This is how I saw the Messenger of Allah, may Allah bless him and grant him peace, doing."