Saeed bin Jubayr led us in prayer at Jam, performing Maghrib as three units with a single iqamah, then he gave the salam, then he prayed Isha as two units. He then mentioned that Abdullah bin Umar had done the same, and he mentioned that the Messenger of Allah ﷺ had done the same.
حدثنا بهز حدثنا شعبة قال أخبرني
صلى بنا سعيد بن جبير بجمع المغرب ثلاثا بإقامة قال ثم سلم ثم صلى العشاء ركعتين ثم ذكر أن عبد الله بن عمر فعل ذلك وذكر أن رسول الله ﷺ فعل ذلك
Its isnad is sahih. Hakam is Ibn Utayba. This hadith belongs to the Musnad of Abdullah bin Umar, and has no connection to the Musnad of Ibn Abbas. Its meaning will come again repeatedly in the Musnad of Ibn Umar, including at 4452, 4460, 4472, 4542, 4598, and see also 2465.