Abu Humayd al-Ansari brought a vessel of milk during the day to the Prophet ﷺ while he was at Baqi. The Prophet ﷺ said, "Why did you not cover it, even by placing a stick across it?"
حدثنا عبد الرزاق أخبرنا سفيان وأبو نعيم قال حدثنا سفيان عن أبي الزبير
جاء أبو حميد الأنصاري بإناء من لبن نهارا إلى النبي ﷺ وهو بالبقيع فقال النبي ﷺ ألا خمرته ولو أن تعرض عليه عودا
Its chain of transmission is sahih. Ahmad did not narrate anything here, but rather he was narrating to his son how eager he was for knowledge, and that he sat at the door of Ibrahim bin Aqeel for a day and more in order to hear from him.