Maliks Muwatta Book 9, Hadith Number 23.
Section : The Prayer of a Traveller when Imam, or Behind an Imam.
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.”