It was narrated from Ibn Wa'lah that he asked Ibn 'Abbas: "We are attacking the Maghrib, and they are people who worship idols, and they have waterskins in which they keep milk and water." Ibn 'Abbas said: "Tanning is purification." Ibn Wa'lah said: "Is this your own opinion, or something that you heard from the Messenger of Allah?" He said: "No, (I heard it) from the Messneger of Allah."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنِي الرَّبِيعُ بْنُ سُلَيْمَانَ بْنِ دَاوُدَ، قَالَ حَدَّثَنَا إِسْحَاقُ بْنُ بَكْرٍ، - وَهُوَ ابْنُ مُضَرَ - قَالَ حَدَّثَنِي أَبِي، عَنْ جَعْفَرِ بْنِ رَبِيعَةَ، أَنَّهُ سَمِعَ أَبَا الْخَيْرِ، عَنِ ابْنِ وَعْلَةَ، أَنَّهُ سَأَلَ ابْنَ عَبَّاسٍ فَقَالَ إِنَّا نَغْزُو هَذَا الْمَغْرِبَ وَإِنَّهُمْ أَهْلُ وَثَنٍ وَلَهُمْ قِرَبٌ يَكُونُ فِيهَا اللَّبَنُ وَالْمَاءُ فَقَالَ ابْنُ عَبَّاسٍ الدِّبَاغُ طَهُورٌ . قَالَ ابْنُ وَعْلَةَ عَنْ رَأْيِكَ أَوْ شَىْءٍ سَمِعْتَهُ مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ بَلْ عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم .
sunnah.com