Shama-il Tirmidhi Chapter 023, Hadith Number 004 (133).
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 has been mentioned in some narrations that he first licked the middle finger, then the sha-haadah finger, then the thumb. It was the noble habit of the master to use these three fingers. The 'ulama have mentioned many benefits in this method (sequence). The first is that the licking of the fingers will run in a manner where it goes to the right. The Sha-haadah finger will be on the right of the middle finger. The second is that the middle finger is long, therefore it will be more contaminated. For this reason it is appropriate to begin with this finger. Khattaabi says: 'Some foolish people do not like to lick the fingers, and think it disgraceful, but they do not reason that the food that is on the finger is the same that they have been eating, there is nothing new on it'. Ibn Hajar says: 'If someone thinks of his own deed as disgraceful, it could be discussed. But to think of any act of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallasm as disgraceful, could be dangerous and may even lead to kufr (disbelief) - Jam'ul Wasaa-il. In reality these things have alot to do with habit. If one has a habit of something, it does not matter, and one will not even take notice of it. This is why if one naturally feels these deeds to be disgraceful, then too one should try to form a habit of it, (and should remind one's self that it is sunnah of Sayyidina Rasulullah (Sallallahu 'Alayhi Wasallam)). Once when this humble servant went to Hijaaz (Saudi Arabia), a few who had not been to India asked me surprisingly and with great astonishment, that we hear that there is a fruit in India known as the Mango. We hear such disgusting things about it that it surprises us. It is put to the mouth and sucked, it is removed then put to the mouth and sucked again, It is again removed looked at, and sucked again. They were describing it with such disgust that it seemed they were about to vomit. An Indian will not feel anything disgusting about it. There are many such examples. One takes a spoonful of custard and puts it into the mouth, then puts the same spoon which has some saliva on it, into the plate again, eating from it a second and third time. There are countless other instances where a person is used to a certain way and manner and does not feel the least disgusted about it.