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The only Israel in Egypt with which I'm familiar is Händel's. It has a tremendous duet for two bass voices, "The Lord is a Man of War". As Mendelssohn died before the tuba was invented, perhaps the piece of which you're writing has an ophecleide part as does his Incidental Music to a Midsummer Night's Dream.

Norm in Bellevue wrote:... As Mendelssohn died before the tuba was invented, perhaps the piece of which you're writing has an ophecleide part as does his Incidental Music to a Midsummer Night's Dream.

Norm, I played Mendelssohn's Symphony No. 5 in May and there was a nice Serpent part that I played on F tuba.

I'm not familiar with the piece the orignal poster asked about and unfortunately I don't have my copy of Daniels with me right now. Sorry.

Weird. Googling around I can only find a few references to this work, but nothing concrete. It seems to exist, but I can only find a few program listings and nothing much else. Groves doesn't seem to mention it.
Have a look as well for psalm cv (105).

Hmmm. I found this entry:

Israel in Aegypt
Oratorio
Musica di Georg Friedrich Händel
Riduzione per soli, coro e pianoforte di Felix Mendelssohn Bartholdy

Which seems to indicate that Mendelssohn did a piano reduction of the Handel work. However, I wonder if you're not really referring to Op. 51, Psalm 114 of Mendelssohn, "Da Israel aus Agypten zog"?

From the Carus-Verlag catalog:

Der 114. Psalm. Da Israel aus Ägypten zog
When Israel came from Egypt’s land op. 51
(1839) (G/E)
Coro SSAATTBB + 2222-4230, Timp, 2 Vl, Va, Vc/Cb
12 min (ed. Bill) /

And if it is the Mendelssohn adaptation of Haendel's Israel in Egypt, there isn't a tuba part in that one either. (I stumbled upon the original score on my way out of the library where I work )