Ka'b bin Maalik Radiyallahu 'Anhu says:
"It was the noble habit of Rasulullah Sallallahu 'Alayhi Wasallamn to use three fingers whilst eating and-he also licked them". (IT)
English reference (IT): Book 23, Hadith 133 (IT)
Arabic reference (IT): Book 24, Hadith 141 (IT)