Praise be to Allah.
Al-Haafiz Ibn Rajab al-Hanbali (may Allah have mercy on him) gave a comprehensive definition of bid‘ah (innovation) when he said:
If anyone introduces something new and describes it as part of religion, when it has no basis at all in religion, then it is misguidance and it has nothing to do with religion.
Jaami‘ al-‘Uloom wa al-Hukam (2/128)
So the pillars (essential parts) of bid‘ah are as follows:
1. It is something newly introduced
2. This newly introduced matter is described as part of the religion
3. This newly introduced matter has no basis or shar‘i proof.
You will find a detailed discussion of this matter in the answer to question no. 118225
Please see also the answers to questions no. 10843 and 128530
The linguistic meaning of maslahah (translated above as (public) interest) refers to something beneficial, whether it has to do with this world or the hereafter.
The linguistic meaning of mursalah is general, pertaining to all.
The technical term maslahah mursalah refers to a benefit concerning which there is no specific evidence of approval or disapproval.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said, explaining the meaning of maslahah mursalah:
This refers to a case where the mujtahid thinks that this action is more likely to bring benefit and there is nothing in Islamic teaching to disallow it.
Majmoo‘ al-Fataawa (11/342, 343).
Here we will quote some scholarly discussions giving guidelines on the differences and similarities between bid‘ahs (innovations) and cases of maslahah mursalah (consideration of the public interest), with which we will conclude our answer and hope that it will be beneficial and useful.
Shaykh Muhammad ibn Husayn al-Jeezaani (may Allah bless him) said:
A. Similarities between bid‘ah and maslahah mursalah:
1. Both bid‘ah and maslahah mursalah have to do with things that did not occur at the time of the Prophet (blessings and peace of Allah be upon him), especially in the case of maslahah mursalah, as in the case of bid‘ahs, they were rare. However there may be some bid‘ahs – although this is rare – that occurred during his time, as in the story of the three people who came to ask about the worship of the Prophet (blessings and peace of Allah be upon him) [and they thought it was too little and that they had to strive harder].
2. Both bid‘ah – as is usually the case – and maslahah mursalah have no specific textual evidence, because all they can use for proof is texts that are general in wording and meaning.
B. Differences between bid‘ah and maslahah mursalah:
1. What makes bid‘ah different is the fact that it only has to do with rituals of worship and similar religious matters, unlike maslahah mursalah, because maslahah mursalah is examined within a framework of rational analysis and thought such that, if (a matter) is presented to people of understanding they would accept it (on a purely rational basis), and it has nothing to do with rituals of worship or similar shar‘i matters.
2. Another difference is that bid‘ah is intended in and of itself by those who do it (and they have no reason beyond that). So they usually seek to draw closer to Allah by doing it, and they never drift away from it. It never occurs to give it up, because they think that their bid‘ah is superior to any argument that could differ from it. This is unlike maslahah mursalah, which is intended as a means to achieve some objectives and it is not done for its own sake. Thus it comes under the heading of means, because it is only prescribed as a means of attaining some of the goals of sharee‘ah. That is indicated by the fact that this action may no longer be valid and will no longer be of any value from a shar‘i point of view, once it has led to a harm that outweighs any benefit (because it is not sought in and of itself). In that case, it is not possible to examine bid‘ah from the same angle as maslahah mursalah.
3. Another difference is that bid‘ah usually leads to an additional burden on the worshipper and making things too strict, unlike maslahah mursalah, which leads to a reduction of burdens and protects people from hardship, or it leads to protection of something that is important to them.
4. Another difference is that bid‘ah is contrary to the aims of sharee‘ah and undermines them, unlike maslahah mursalah which – in order for it to be regarded as having value from a shar‘i point of view – must be in harmony with the aims of sharee‘ah and must serve to achieve them, otherwise it is not to be allowed.
5. If a maslahah mursalah did not occur at the time of the Prophet (blessings and peace of Allah be upon him), that was only because the reason for it was not there, or there were some impediments that prevented it from occurring. This is unlike bid‘ah; if a bid‘ah did not happen at the time of the Prophet (blessings and peace of Allah be upon him), that is in spite of the fact that there may have been reasons or motives for it to occur and no impediments.
To sum up: if the conditions of maslahah mursalah are met, it is the opposite of bid‘ah and differs from it. Maslahah mursalah cannot be classified as bid‘ah, otherwise – if that were to happen – it would become invalid and would no longer be called maslahah mursalah; rather it would be called an invalid maslahah or a harmful maslahah.
Qawaa‘id Ma‘rifat al-Bida‘ (p. 19,20)
And Allah knows best.