I lay out here what has been suggested in comments by @jbowman.
Let a constant a≥0. Let Yi follow an Exp(1) and consider Zi=Yi−a. Then
Pr(Zi≤zi∣Yi≥a)=Pr(Yi−a≤zi∣Yi≥a)
⟹Pr(Yi≤zi+a∣Yi≥a)=Pr(Yi≤zi+a,Yi≥a)1−Pr(Yi≤a)
⟹Pr(a≤Yi≤zi+a)1−Pr(Yi≤a)=1−e−zi−a−1+e−ae−a=1−e−zi
which is the distribution function of Exp(1).
Let's describe this: the probability that an Exp(1) r.v. will fall in a specific interval (the numerator in the last line), given that it will exceed the interval's lower bound (the denominator), depends only on the length of the interval and not on where this interval is placed on the real line. This is an incarnation of the "memorylessness" property of the Exponential distribution, here in a more general setting, free of time-interpretations (and it holds for the Exponential distribution in general)
Now, by conditioning on {Yi≥a} we force Zi to be non-negative, and crucially, the obtained result holds ∀a∈R+. So we can state the following:
If Yi∼Exp(1), then ∀Q≥0:Zi=Yi−Q≥0 ⟹ Zi∼Exp(1).
Can we find a Q≥0 that is free to take all non-negative real values and for which the required inequality always holds (almost surely)? If we can, then we can dispense with the conditioning argument.
And indeed we can. It is the minimum-order statistic, Q=Y(1), Pr(Yi≥Y(1))=1. So we have obtained
Yi∼Exp(1)⟹Yi−Y(1)∼Exp(1)
This means that
Pr(Yi−Y(1)≤yi−y(1))=Pr(Yi≤yi)
So if the probabilistic structure of Yi remains unchanged if we subtract the minimum order statistic, it follows that the random variables Zi=Yi−Y(1) and Zj=Yj−Y(1) where Yi,Yj independent, are also independent since the possible link between them, Y(1) does not have an effect on the probabilistic structure.
Then the sum ∑ni=1(Yi−Y(1)) contains n−1 Exp(1) i.i.d. random variables (and a zero), and so
∑i=1n(Yi−Y(1))∼Gamma(n−1,1)