Muwatta Malik, Book 18, Hadith 30 (USC-MSA)

Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to be cupped while he was fasting. Nafi said, "He later stopped doing that, and would not be cupped when he was fasting until he had broken the fast."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ كَانَ يَحْتَجِمُ وَهُوَ صَائِمٌ - قَالَ - ثُمَّ تَرَكَ ذَلِكَ بَعْدُ فَكَانَ إِذَا صَامَ لَمْ يَحْتَجِمْ حَتَّى يُفْطِرَ ‏.‏

sunnah.com

also referenced under: Book 18, Hadith 664 (In-Book)