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". (RU)
English reference (RU): Book 23, Hadith 133 (RU)
Arabic reference (RU): Book 24, Hadith 141 (RU)