مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2175
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from ‘Ikrimah: Ibn 'Abbas (رضي الله عنه) told us; The Messenger of Allah (ﷺ) did ghusl for janabah, and when he came out he saw a spot on his left shoulder that the water had not reached, so he took some [water] from his hair and wetted it, then he went ahead and prayed.
Musnad Ahmad 2180Its isnad is da'eef jiddan because Alibin Asim is weak]