Ethics of the Fathers (Pirkei Avot) 1:14:
He [Hillel] would also say: If I am not for myself, who is for me? And if I am only for myself, what am I? And if not now, when?
I made this to always remind me of Hillel´s saying, especially when tempeted to be lazy. My father and grandfathers used to say:
Was Du heute kannst besorgen, das verschiebe nicht auf Morgen… (“What you can do today, don´t delay to tomorrow…”)
Pirkei Avot is one of 63 short books that make up the Mishna. It´s the only one, which is not a legal text. The quotes found in Pirkei Avot generally are spiritual and edifying, but they can also be practical.