Muwatta Malik, Book 40, Hadith 1500 (In-Book)

Malik related to me from Nafi that Abdullah ibn Umar made two of his slave-girls mudabbara, and he had intercourse with them while they were mudabbara.

حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، دَبَّرَ جَارِيَتَيْنِ لَهُ فَكَانَ يَطَؤُهُمَا وَهُمَا مُدَبَّرَتَانِ ‏.‏

sunnah.com

also referenced under: Book 40, Hadith 4 (USC-MSA)