Opportunity passes like a cloud; count the chances for good as a gain.
الفرصة تمر مر السحاب، فانتهزوا فرص الخير
Al-fursatu tamurru marra's-sahâb, fantahizû furasa'l-khayr
Ali, Nahj al-Balagha, saying 21 (Subhi Salih edition, p. 471; saying 20 in the Abduh edition). Nahj al-Balagha is a compilation by al-Sharif al-Radi (d. 1015), without chains of transmission.
A cloud comes, gives shade, and passes.
It does not wait.
So it is with opportunity.
It knocks at the door; if the door is not opened, it leaves.
Ali speaks of the chances for good.
A kindness you could do
will not wait in the same place tomorrow.
Rest while there is shade,
sow while there is rain.