Muwatta
Malik
موطأ مالك
09
Shortening the Prayer
كتاب قصر الصلاة فى السفر
Chapter 6: The Prayer of a Traveller when Imam, or Behind an Imam
Muwatta Malik 353
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to pray four Rak'ahs 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 .