Aslam reported: Umar, may Allah be pleased with him, performed ablution from the water of a Christian women in the jar of a Christian woman.
Source: al-Sunan al-Kubrá lil-Bayhaqī 129
Grade: Sahih (authentic) according to Al-Nawawi
Al-Shafi’i said, “There is nothing wrong with ablution from the water of an idolater or what is left over from his ablution, as long as it is known there is no impurity in it, because water is pure in itself, wherever it is, unless it is known that impurity is mixed with it.”
Source: al-Umm 1/21
عن أسلم أَنَّ عُمَرَ رَضِيَ اللهُ عَنْهُ تَوَضَّأَ مِنْ مَاءِ نَصْرَانِيَّةٍ فِي جَرَّةِ نَصْرَانِيَّةٍ
129 السنن الكبرى للبيهقي
1/263 المحدث النووي خلاصة حكم المحدث صحيح في المجموع
قَالَ الشَّافِعِيُّ وَلَا بَأْسَ بِالْوُضُوءِ مِنْ مَاءِ الْمُشْرِكِ وَبِفَضْلِ وُضُوئِهِ مَا لَمْ يَعْلَمْ فِيهِ نَجَاسَةً لِأَنَّ لِلْمَاءِ طَهَارَةً عِنْدَ مَنْ كَانَ وَحَيْثُ كَانَ حَتَّى تُعْلَمَ نَجَاسَةٌ خَالَطَتْهُ
1/21 كتاب الأم للشافعي