It is narrated from Zayd bin Aslam (may Allah have mercy on him) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) drank some milk, and it amazed him. He asked the one who had given it to him to drink: Where did you get this milk from? He told him that he had gone to a watering place, and he named that watering place, and there were sadaqah goats there which they were watering. They milked some of their milk for me, and I put it in this vessel of mine. Sayyiduna Umar (may Allah be pleased with him) put his finger (into his mouth) and vomited it out.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الصدقات / 13164