مُوَطَّأُ مَالِكٍ
Muwatta Malik 118
Purity
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ لاَ بَأْسَ أَنْ يُغْتَسَلَ بِفَضْلِ الْمَرْأَةِ مَا لَمْ تَكُنْ حَائِضًا أَوْ جُنُبًا .
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "There is no harm in doing ghusl with water that has been used by one's wife as long as she is not menstruating or in a state of major ritual impurity (junub)."
Arabic/English book reference: Book 2, Hadith 88