Yahya related to me from Malik from Zayd ibn Aslam that hisfather said, "Abdullah ibn al-Arqam said, 'Show me a riding-camelwhich the amir al-muminim can give me to use.' I said, 'Yes. One ofthe sadaqa camels.' Abdullah ibn al-Arqam said, 'Would you want astout man on a hot day to wash for you what is under his lower garmentand its folds, and then give it to you to drink?' I was angry andsaid, 'May Allah forgive you! Why do you say such things to me?'Abdullah ibn al-Arqam said, 'Sadaqa is the impurities of people whichthey wash off themselves.' "
USC-MSA web (English) reference: Book 58, Hadith 15
Arabic reference: Book 58, Hadith 1858