Don’t say “When I have time I will learn,” lest you never have time.

[וְאַל תֹּאמַר לִכְשֶׁאִפָּנֶה אֶשְׁנֶה, שֶׁמָּא לֹא תִפָּנֶה:]

Hillel (1st C. BC-1st C. AD) Jewish sage, rabbi [הלל]
Mishna, Seder Nezikin [Order of Damages], Pirkei Avot [Chapters of the Fathers] 2:4
    (Source)

(Source (Hebrew)). Alternate translations:

Say not, When I have leisure I will study; perchance thou mayest not have leisure.
[tr. Taylor (1897)]

Say not: ‘when I shall have leisure I shall study;’ perhaps you will not have leisure.
[tr. Gorfinkle (1913)]

Say not: ‘when I shall have leisure I shall study;’ perhaps you will not have leisure.
[tr. Kulp]

Do not say: When I can free myself [of my affairs] I shall learn (Torah); perhaps you will not free yourself.
[tr. Shraga Silverstein]

Do not say, "When I will be available I will study [Torah]," lest you never become available.
[Open Mishnah]

Do not say "When I have leisure, I will study," perhaps you will not have leisure.
[Source]

Say not, "When I have free time I shall study"; for you may perhaps never have any free time.