مُسْنَدُ أَحْمَدَ
Musnad Ahmad 1880
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn "Abbas said: The Messenger of Allah (ﷺ) used to pray two rak"ahs at night, then when he finished he would use the siwak.
Musnad Ahmad 1881Sahih (Darussalam) []