Bukhari

Hadith Commentary:
This hadith has already appeared with a connected chain in the Book of Maghazi.

In this chapter, Imam Bukhari rahimahullah has compiled those ahadith in which the word "if" (agar) is mentioned, from which it is understood that saying "if" is not absolutely prohibited. And in the other hadith in which it is stated, "Avoid 'if' and 'but'," that is restricted to specific situations; meaning, when one intends to do a good deed and has the ability to do so, then he should carry it out. In this, he should not bring up excuses or say "if" and "but."

Secondly, when a calamity befalls or some loss occurs, one should understand it as being from Allah's decree (qadar) and His will. In this situation as well, bringing up "if" and "but" and saying, "If we had done such-and-such, then this calamity would not have occurred," is prohibited, because in this there is a lack of trust in Allah's decree and reliance upon one's own planning.

Hadith Commentary:
1.
What is meant here is not the transfer of family lineage, because transferring family lineage to another side is forbidden. Moreover, it is a fact that, in terms of lineage, the Messenger of Allah (sallallahu alayhi wa sallam) himself is superior to the Ansar. In this, the Messenger of Allah (sallallahu alayhi wa sallam) has stated: After migration, there is no virtue greater than supporting Islam, and it is for this very reason that the Ansar attained such a lofty status.

2.
The summary of the matter is that, had I not been given superiority over the Ansar due to migration, I would have been among the Ansar.

3.
In any case, Imam Bukhari (rahimahullah) has established from these ahadith that the prohibition of using the word (لوْ, "if") in certain ahadith is not absolute. If it were so, neither the Messenger of Allah (sallallahu alayhi wa sallam) nor his noble Companions (radi Allahu anhum ajma'in) would have used it. This prohibitive ruling applies to specific situations, the details of which are as follows:
©.
If one intends to do a good deed and has the ability to do it, then one should not fall short in its fulfillment due to something else, nor should one use "if" or "but" as an excuse for negligence in such matters.
©.
If something of the worldly affairs is lost or some harm occurs due to a calamity, then one should be content with its decree and destiny, and not object to Allah’s decree by saying "if" or "but."
If falsehood is involved in these, then it becomes an extremely grave matter, for example: The hypocrites once said: "If we had the strength, we would certainly have gone out with you." () In another place, they said: "If they had obeyed us, they would not have been killed there." () By doing so, it demonstrates distrust in Allah’s decree and reliance upon one’s own planning.
(Fath al-Bari: 13/282)