Sahih al-Bukhari — One-fifth of Booty to the Cause of Allah (Khumus) — Hadith No: 3153
Translation
Narrated `Abdullah bin Mughaffal:While we were besieging the fort of Khaibar, a person threw a leather container containing fat, and I ran to take it, but when I turned I saw the Prophet (standing behind), so I felt embarrassed in front of him