The first answer is that this hadith contains a scribal error, and that its original wording was: “He will prosper, by Allah.” In early times, however, they did not use diacritical dots; and “by his father” resembles “by Allah” in that both contain two upright strokes and a hā’; but the two strokes were shortened and the diacritical dots omitted, so it became “by his father.” This is incorrect, for the basic presumption is that there has been no scribal error. Moreover, it opens a dangerous door regarding the narrators: whenever we find something unacceptable, we could simply say, “This is a scribal error.”
The second answer is that this occurred before swearing by one’s fathers was prohibited. In the beginning, this was widespread and common, and people had become accustomed to it; so its prohibition came later, just as the prohibition of wine came later, for it was not forbidden until the sixth year after the Hijrah. Likewise, hijab was not made obligatory until the sixth year after the Hijrah. For when something is familiar, it is difficult for the soul to abandon it at the outset. They therefore said that the Sacred Law left them upon this practice because it was familiar to them, and when faith had become firmly established in their hearts, it prohibited it. Thus, the Prophet’s ﷺ oath, “by his father,” would have occurred before the prohibition, and we would then say that it was abrogated.
However, one of the conditions for abrogation is knowledge of the chronology. Mere reasoning does not establish what came earlier and what came later; this is insufficient. Rather, we must know that it came later. Accordingly, the claim of abrogation is also weak.
The third answer is that this is something uttered unintentionally, and thus falls under meaningless oaths. Allah, Exalted is He, says:
“Allah will not impose blame upon you for what is meaningless in your oaths.” [al-Ma’idah: 89]
If we were to assume that people had become accustomed to this, then we would leave them as they are. Accordingly, we would not forbid those who have become accustomed to swearing by the Prophet ﷺ, because it simply comes upon their tongues. A man once came to me seeking a legal ruling from me—