Muwatta Imam Malik Book of Shortening the Prayer, Hadith 22

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.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يُصَلِّي وَرَاءَ الإِمَامِ بِمِنًى أَرْبَعًا فَإِذَا صَلَّى لِنَفْسِهِ صَلَّى رَكْعَتَيْنِ .