مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2269
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn 'Abbas (رضي الله عنه) that The Prophet (ﷺ) got up at night to pray. So I [Ibn 'Abbas] got up and did wudoo’, then I stood on his left, but he pulled me and made me stand on his right. He prayed thirteen rak'ahs in which the length of standing was equal.
Musnad Ahmad 2276Its isnad is saheeh]