Muwatta Malik Book 9, Hadith 25
Yahya related to me from Malik from Ibn Shihab that Safwan said,
"Abdullah ibn Umar used to come and visit Abdullah ibn Safwan and he
would pray two rakas with us, and when he went away we would stand and
complete the prayer."
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to pray four rakas behind the imam at Mina, and when he prayed by himself he would pray two rakas.
Yahya related to me from Malik from Nafi that Abdullah ibn Umar never used to pray anything with the fard prayer, either before it or after it, while travelling, except in the depths of the night. He would pray on the ground or on his mount, whichever way it was facing.