Sahih Bukhari
One-fifth of Booty to the Cause of Allah (Khumus)
Volume: 004 / Book Number: 053 / Hadith Number: 381
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.