Hadith 503510
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يُصَلِّي وَرَاءَ الإِمَامِ بِمِنًى أَرْبَعًا فَإِذَا صَلَّى لِنَفْسِهِ صَلَّى رَكْعَتَيْنِ .
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.
Reference Muwatta Malik 503510
USC-MSA web (English) reference : Book 9, Hadith 22