Saying Bismillah Ar-Rahman Ar-Rahim before Eating
Question

I wanted to ask whether it is allowed to say "Bismillahir-Rahmanir-Rahim" instead of "Bismillah" before one eats. There was a discussion between a sister and me, and I said it is a Bida, but the sister insisted that there is nothing wrong with that. What is the right ruling on this issue?
Praise be to Allah.Significance of saying Bismillah before eating
The phrase that is prescribed for mentioning Allah when eating is to say “Bismillah ” (In the name of Allah), because of the report narrated by `A’ishah (may Allah be pleased with her), that the Prophet (blessings and peace of Allah be upon him) said: “When one of you eats some food, let him say Bismillah , and if he forgets to do so at the beginning, let him say Bismillah fi awwalihi wa akhirihi (In the name of Allah at the beginning and end). (Narrated by At-Tirmidhi, 1781; classed as authentic by Shaykh Al-Albani (may Allah have mercy on him)) 
Bismillah or Bismillah Ar-Rahman Ar-Rahim?
The scholars (may Allah have mercy on them) differed concerning saying Bismillah Ar-Rahman Ar-Rahim (in the name of Allah, the Most Gracious the Most Merciful). Most of the scholars are of the view that there is nothing wrong with this addition. 
Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) said: If one says when eating, Bismillah Ar-Rahman Ar-Rahim, that is good and more perfect. (End quote from Al-Fatawa Al-Kubra, 5/480) 
It says in Al-Mawsu`ah Al-Fiqhiyyah, (8/92): The jurists are of the view that mentioning the name of Allah when beginning to eat is a Sunnah, and the phrase to be used is Bismillah or Bismillah Ar-Rahman Ar-Rahim. (End quote) 
An-Nawawi (may Allah have mercy on him) said: It is more important to note the phrase to be used when mentioning the name of Allah. … The best is to say Bismillah Ar-Rahman Ar-Rahim, but if one says Bismillah , that is sufficient and fulfills the Sunnah. (End quote from Al-Adhkar, 1/231)
Al-Hafidh ibn Hajar (may Allah have mercy on him) followed that with his comment: I have not seen any particular evidence to support what he claims about it being better. (End quote from Fat-h Al-Bari) 
Shaykh Al-Albani (may Allah have mercy on him) said: And I say: There is nothing better than his Sunnah (blessings and peace of Allah be upon him). The best of guidance is the guidance of Muhammad (blessings and peace of Allah be upon him) and if nothing is proven concerning mentioning the name of Allah when eating except to say “Bismillah,” then it is not permissible to add anything to it, let alone suggest that the longer phrase is better than it, because saying such a thing is contrary to what is referred to in the Hadith: “The best of guidance is the guidance of Muhammad (blessings and peace of Allah be upon him).” (End quote from As-Silsilah As-Sahihah, 1/343)
Based on this, it is better to limit it to saying Bismillah when starting to eat, and not add anything to it. But if one does add to it by saying Bismillah Ar-Rahman Ar-Rahim, there is nothing wrong with that according to the majority of scholars. 
For more details, please see the following answers: 22186 , 173672 , 175312 .
And Allah knows best.