by Imam Malik, translation by A'isha `Abdarahman at-Tarjumana and Ya`qub Johnson , Volume: Shortening the Prayer
Chapter: The Prayer of a Traveller when Imam, or Behind an Imam
Yahya related to me from Malik from Ibn Shihab from Salim ibn Abdullah from his father that Umar ibn al-Khattab, when he went to Makka, used to lead them in prayer and do two rakas and then say, "People of Makka,complete the prayer, we are a group travelling."
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 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 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."