مُوَطَّأُ مَالِكٍ
Muwatta Malik 351
Shortening the Prayer
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.
Arabic/English book reference: Book 9, Hadith 22