مُوَطَّأُ مَالِكٍ
Muwatta Malik 409
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ رَبِيعَةَ بْنِ أَبِي عَبْدِ الرَّحْمَنِ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ إِذَا جَاءَ الْمَسْجِدَ - وَقَدْ صَلَّى النَّاسُ - بَدَأَ بِصَلاَةِ الْمَكْتُوبَةِ وَلَمْ يُصَلِّ قَبْلَهَا شَيْئًا .
Yahya related to me from Malik from Rabia ibn Abi Abd ar-Rahman that if Abdullah ibn Umar arrived at the mosque and the people had already prayed, he would begin with the obligatory prayer and not pray anything before it.
Arabic/English book reference: Book 9, Hadith 78