Muwatta Imam Malik Book of Shortening the Prayer, Hadith 78

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.

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 .