"If God has written happiness for you,
No one can steal it from your heart.
And if He has written for your heart to break,
No one can heal it but Him..."
"Hearts desire, but the Lord decrees."
لا ينفع حذر من قدر
"No precaution can help against fate."
المكتوب على الجبين لا بد أن تراه العين
"What is written will be fulfilled." (Literally, "what is written on the forehead must be seen by the eye.")
A Bonus Quote: (I was not able to track down the Arabic, but this is said to be in al-Ghunya li Talibi Tariq al-Haqq by Sh. Abd al-Qadir al-Jilani. If you have the Arabic text please let me know!)
'Umar [ibn al-Khattab] (may Allah be well pleased with him) used to quote these two verses [of Arabic poetry] as an instructive example:
"Go easy on yourself, for the outcome of all affairs is determined by God's decree.
If something is meant to go elsewhere, it will never come your way, but if it is yours by destiny, from you it cannot flee."