Muwatta
Malik
موطأ مالك
09
Shortening the Prayer
كتاب قصر الصلاة فى السفر
Chapter 4: The Prayer of a Traveller when Undecided whether to Remain in a Place or Not
Muwatta Malik 349
Yahya related to me from Malik from Nafi that Ibn Umar stayed in Makkah for ten nights, shortening the prayer, except when he prayed it behind an imam, in which case he followed the imam's prayer.
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، أَقَامَ بِمَكَّةَ عَشْرَ لَيَالٍ يَقْصُرُ الصَّلاَةَ إِلاَّ أَنْ يُصَلِّيَهَا مَعَ الإِمَامِ فَيُصَلِّيهَا بِصَلاَتِهِ .