Mishnah 14 - Chapter 1 - Pirkei Avot

Hu hayah omer, im ein ani li, mi li. Ucheshe'ani le'atzmi, mah ani. Ve'im lo achshav, eimatai.

He [also] used to say: If I am not for myself, who is for me? But if I am for my own self [only], what am I? And if not now, when?

He was wont to say: If I do not [acquire merit] for myself, who [will acquire it] for me? And [even] if I do [acquire it] for myself, what [is it, relative to what] I am [obliged to acquire (i.e., relative to my potential)?] And if not now [i.e., in this world], then when? [For after I die, I can no longer acquire merit. Alternately: If not now (in my youth), then when? (Perhaps, in my old age, I will no longer be able to acquire it)].