Musnad Ahmad 456:4

Ya'la and Muhammad, the two sons of 'Ubayd, said: Muhammad ibn Ishaq narrated to us, from Muhammad ibn Ibrahim, from Abu Salamah ibn 'Abd al-Rahman, from Zayd ibn Khalid al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Were it not that I would burden—and Muhammad said: Were it not that it would burden—my nation, I would have delayed the 'Isha prayer until one-third of the night, and I would have commanded them to use the siwak at every prayer."