Eating food that fell on floor is good conduct, not an obligation
17-1-2015 | IslamWeb
Question:
As-salaamu 3laikum, I heard that the Messenger of Allah (salla Allahu 3laihi wa salam) said that we should pick up any food that falls on the floor and eat it. Can you give me some more information about this, such as whether this is fardh or mustahabb, and whether there could be any valid excuse for not doing it, like if the food falls in a place where you know there is harmful bacteria or if you are in a situation where you are more at risk from bacteria, for example, pregnancy. Or should we follow this command in all circumstances? Jazaak Allahu khair
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Among the norms of conduct of eating that the Sunnah presents is as Muslim reported from Jaabir 
: I heard the Prophet, sallallaahu ‘alayhi wa sallam, saying, "The devil is present with each of you in everything he does; he is present when you are eating; so if any one of you drops food he should remove any dirt on it and eat it and not leave it for the devil; and when he finishes eating he should lick his fingers, for he does not know in which portion of his food the blessing lies." [Muslim]
Moreover, Anas said: "When the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, ate, he would lick his three fingers, and he said: 'If one of you drops a piece (of food) then let him remove any harm (dirt) from it and eat it, and not leave it for the devil;' and he commanded us to wipe the plate clean. He said: 'for you do not know in which portion of your food the blessing lies.’" [Muslim]
The command to pick up the food that falls on the floor and eat it is Mustahabb (recommended), not Fardh (obligatory), to be mindful of the favor of Allaah and be distanced from the attitudes of the prideful. Tuhfat Al-Ahwathi, a commentary on the Sunan of At-Tirmithi, reads: "Leaving the food that falls is (described as) leaving it for the devil is because it involves neglecting the favor of Allaah and undervaluing it. Moreover, it is the practice of the prideful. People's unwillingness to pick up the food that has fallen on the floor and eat it is often from their pride; and this is the devil’s work."
An-Nawawi 
said: "The Hadeeth implies that it is recommended to eat food that falls on the ground after removing any dirt that got onto it; this is of course if it has not fallen on an impure place. If it has fallen on an impure place, then it becomes ritually impure as well and must be washed if possible; otherwise, it should be fed to an animal and not left for the devil.'"
Some scholars, on the other hand, held that the command here implies obligation; but even according to this view, if it is feared that eating this food would cause harm to the person, then one should not eat it so as to ward off the potential harm. The Prophet, sallallaahu ‘alayhi wa sallam, said: "There should be neither harm nor reciprocal harm.” [Maalik and Ibn Maajah]
Please refer to Fataawa 116244 and 90428 about some of the norms of conduct for eating and drinking.
Allaah Knows best.