Muwatta Malik Book 9, Hadith 24
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 the same as that to me from Malik from Zayd ibn Aslam from his father from Umar ibn al-Khattab.
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."