Did Muslims swear by "the beard of the Prophet"?

Question

In Western historical fictions, Muslims (from the Middle East) are often depicted to swear by "the beard of the Prophet".

"By the beard of the prophet, we will not offend you; but unless you withdraw from the bars and allow us to drop the bridge, your lives will not be safe many minutes."

-- a Corsair in Isidora

or,

the Caliph likewise swore by the beard of the Prophet that the slave girl would be his personal property that very night.

-- attributed to work of playwright Friedrich Dürrenmatt

or,

I'll drop it down to the bottom and they can fish for it themselves, which they can't do. Tell him I swear that, by - by the Koran or the beard of the Prophet or whatever they swear by!

-- British naval captain Hornblower sending a message to a Turk in Hornblower and the Atropos

and more. Is this mode of swearing supported by historical evidence at all? I haven't heard a modern-day Muslim did it, and according to this Islam StackExchange question it's even prohibited in Islam.

Answer 1

This is a classic case of false depiction which is not uncommon in fiction. The Hadith (Sayings of Muhammad) quoted in the question you linked is correct and Muslims follow the "Sahih Hadiths" (Which are the ones they consider correct and are cited from either Bukhari, Tirmazi, Ibn Maja or Nisai, the four "correct books" of Hadith). To quote it again:

Umar ibn Al-Khattab narrated that the Prophet said: Verily! Allah forbids you to swear by your fathers. If one has to take an oath, they should swear by Allah or else keep silent.'

So it is clear that Muslims are forbidden to swear unless they swear by God. And speaking by my own experience, I have never heard any Muslim swearing by beard of Prophet nor have I read about it in accounts of many Muslim Generals, Rulers as I have deep interest in Oriental history. Even after specifically searching for the term I could not find it being used by any historical figure at all.

But I have seen Muslims swearing by Quran however even though that itself isn't permitted in Islam. I have also seen Muslims swearing by their mothers or graves of their fathers. But such behavior is frowned upon and a habit to swear is discouraged at all levels.

In conclusion, no, there is no evidence supporting that any Muslim in recorded history used that swear.

Answer 2

Kazantzakis' historical fiction Freedom and Death, written in the 1940s but recalling his childhood in the 1880s, features both Christians making scatological references to the "prophet's beard", and Muslims swearing by the prophet's beard; that suggests to me the reference isn't a pure Western invention (Greeks under Ottoman rule aren't quite Western), although Kazantzakis did read plenty of Western European fiction before writing his novel:

Χότζα, φώναξε, στο Μεγάλο Κάστρο εγώ κάνω κουμάντο, θα σου βάλω, μά τα γένια του Προφήτη, στουμούχα, ως καθώς βάζουν στους δαγκανιάρηδες σκύλους, να σωπάσεις! "Imam, [the pasha] yelled, I'm the boss of Kastro [= Iraklion]: by the beard of the prophet, I'll gag you like a rabid dog."

The scatological references by Greek Christians ("I spit on, I crap on") are frequent and old enough to suggest to me they were authentic; but they don't prove that Muslims actually swore by the prophet's beard.

https://www.redensarten-index.de/suche.php?suchbegriff=~~Beim%20Barte%20des%20Propheten!&suchspalte%5B%5D=rart_ou says the saying is documented in the West since the 18th century (which includes Mozart), but is equivocal about whether it is authentic, or a misconstrual of generic swearing by beards.

M. Zwemer's 1948 article "Hairs of the Prophet" (which is cited in books on Google Books, so it's not a modern confection) claims that

The sanctity of Mohammed’s beard as token of manhood and dignity is recognized in common oaths. Even as the Arabs swear by their own lives or by their beards (walahyeti), so more solemnly the Moslem community swears by the beard of their Prophet (lahyet al-nabi). One hears this oath everywhere in the Near East.

I hesitate to call this definitive evidence, though, because searches for Arabic lahyet al-nabi/ لحية النبي or Turkish sakalı şerif don't turn anything that looks like an oath on Google (at least not in the first 3 pages' worth that I ran through Google Translate). I'd have expected more if the phrase was in current use even in the 19th century.

There are travelogues from the 1830s through 1850s by Europeans that clearly cite Muslims swearing by the prophet's beard: 1833 Palestine, 1836 Istanbul, 1855 Saudi Arabia (Richard Burton).

The problem here is that both Islamic and Christian sources can be called into quesstion. Islamic sources would censor out blasphemous references. Christian sources would exoticise Muslim oaths. I find the 1830s Christian testimony convincing, as travelogues rather than historical fiction; but @NSNoob does not, and I appreciate why.

The best scholarly evidence I have found is this recent article on Jordanian Arabic oaths. There's plenty of swearing by the prophet, and there's plenty of swearing by beards; but the source doesn't mention any swearing by the prophet's beard.