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