Muwatta Malik, Book 28, Hadith 1130 (In-Book)

Yahya related to me from Malik that he had heard that Umar ibn al-Khattab gave his son a slave-girl and said, "Do not touch her, for I have uncovered her."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ، وَهَبَ لاِبْنِهِ جَارِيَةً فَقَالَ لاَ تَمَسَّهَا فَإِنِّي قَدْ كَشَفْتُهَا ‏.‏

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also referenced under: Book 28, Hadith 36 (USC-MSA)