Firstly: 

Al-Bukhaari (4979) narrated that Abu Juhayfah (may Allah be pleased with him) said: The Messenger of Allah (blessing and peace of Allah be upon him) said: “I do not eat whilst reclining.” 

What is meant by reclining is any manner of sitting in which one is too comfortable and relaxed, because that may tempt him to eat a great deal, which is blameworthy according to sharee‘ah. 

Hence an-Nawawi said: 

What this means is: I do not eat like one who wants to eat a great deal and sits comfortably; rather I sit like one who is going to get up quickly and I eat little.

End quote from Sharh Muslim

Al-Haafiz (may Allah have mercy on him) said: There was a difference of opinion concerning the manner of reclining (referred to in the hadeeth). It was said that it refers to sitting in a comfortable and relaxed manner, no matter form it may take, or that it refers to reclining on one’s side, or that it refers to resting one’s left hand on the ground. Al-Khattaabi said: The common folk think that eating whilst reclining means eating lying on one’s side, but that is not the case. Rather it refers to sitting on a cushion beneath one. What the hadeeth means is I do not eat sitting on a cushion like one who wants to eat a great deal of food. I only eat enough to keep me going and hence I sit like one who is going to get up quickly.

End quote from Fath al-Baari (9/541). See: Ma‘aalim as-Sunan by al-Khattaabi (2/242); Zaad al-Ma‘aad by Ibn al-Qayyim, 4/202 

Al-Qaari said in al-Marqaah: It was narrated in ash-Shifa’ from some commentators that they interpreted sitting in a comfortable and relaxed manner to eat as referring to sitting cross-legged on a cushion, because this leads to eating a lot of food and makes one arrogant.

End quote from ‘Awn al-Ma‘bood Sharh Sunan Abi Dawood, 10/244 

Secondly: 

With regard to the manner of sitting when one wants to eat, Muslim narrated in his Saheeh (3807) that Anas ibn Maalik said: I saw the Prophet (blessing and peace of Allah be upon him) kneeling, eating a date. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: This means kneeling with the feet upright, resting on one’s heels…. The Prophet (blessing and peace of Allah be upon him) ate like that so that he would not become too comfortable and eat too much, because usually when a person sits in this manner, he does not feel too comfortable, so he does not eat a lot. If he is not comfortable he will never eat a lot, but if he is comfortable he will eat a lot. This is what usually happens…

End quote from Sharh Riyaadh as-Saaliheen

Al-Haafiz (may Allah have mercy on him) said: What is mustahabb when sitting to eat is to kneel down, with the top of the feet resting on the floor, or to sit with the right knee up, resting on the left foot.

End quote from Fath al-Baari. 

These are the three postures for eating: 

1.kneeling with the feet upright, resting on one’s heels

2.kneeling with the top of the feet resting on the floor

3.Sitting with the right knee up, resting on the left foot.

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: But it is better not to sit comfortably lest that be a cause of eating a lot, and eating a lot is not appropriate.

End quote from Sharh Riyadh as-Saaliheen 

And Allah knows best.