Hadith 344
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، أَقَامَ بِمَكَّةَ عَشْرَ لَيَالٍ يَقْصُرُ الصَّلاَةَ إِلاَّ أَنْ يُصَلِّيَهَا مَعَ الإِمَامِ فَيُصَلِّيهَا بِصَلاَتِهِ .
Yahya related to me from Malik from Nafi that Ibn Umar stayed in Makka for ten nights, shortening the prayer, except when he prayed it behind an imam, in which case he followed the imam's prayer
Mauquf Sahih (Salim al-Hilali)
Muwatta Malik 344 · Book 9, Hadith 19