Muwatta Malik, Book 7, Hadith 33 (USC-MSA)

Yahya related to me from Malik from Abd ar-Rahman ibn al-Qasim that al-Qasim ibn Muhammad had done the same as Ibn Umar.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ الْقَاسِمِ، عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ، أَنَّهُ صَنَعَ مِثْلَ الَّذِي صَنَعَ ابْنُ عُمَرَ ‏.‏

sunnah.com

also referenced under: Book 7, Hadith 287 (In-Book)