I have almost always seen the word Tefillah and I suppose Tefillot, spelt with no Yud.
Tav, Feh, Lamed, Vav, Tav. (Tefillot)
Tav, Feh, Lamed, Heh (Tefillah)
But the Sacks singers siddur spells it with a yud (this is clear on its front cover, and presumably consistently throughout. I see it for example in tefillat haderech(prayer for rain) and tefillat tal(prayer for dew), all with a yud. The Singers siddurim before that spelt it with no yud. I have heard that this(with a yud, as the Sacks Singers does) is a spelling mistake "slap bang"(as the brits would say) on the front cover, and was too late for them to change it once it was published.
Is anybody brave enough to say this is a spelling mistake(can one even say that with great certainty?), or point to where else it is ever spelt with a yud? or confirm if it's ever or never spelt with a yud in tenach or other jewish sources?