A 400-year-old Arabic poem: Let the days do what they will

Let the days do what they will,
and keep an easy heart when trouble falls.
If your heart is a contented one,
you and the owner of the world are equal.

دَعِ الأَيّامَ تَفعَلُ ما تَشاءُ
وَطِبْ نَفساً إِذا نَزَلَ البَلاءُ
إِذا ما كُنتَ ذا قَلبٍ قَنوعٍ
فَأَنتَ وَمالِكُ الدُنيا سَواءُ

Anonymous. The earliest witness in this corpus records it as 'by someone' (li-ba'dihim); the attribution to al-Shafi'i (d. 204 AH) appears only in anthologies of the last century.

Quoted in:
• Baha al-Din al-Amili (d. 1031 AH), al-Kashkul, vol. 2, p. 226 [shamela:1571] — gives three bayts under the heading li-ba'dihim, 'by someone' - no poet named - then objects to the third: 'No, by God. The contented man and the owner of the world are NOT equal; the contented man is the less sorrowful, the easier in himself, the more settled in his eye.'
• Ahmad al-Hashimi (d. 1362 AH), Jawahir al-Adab fi Adabiyyat wa-Insha Lughat al-Arab, vol. 2, p. 426 [shamela:21505] — the earliest NAMED attribution found in the corpus - 'and the Imam al-Shafi'i said' - heading a poem now grown to eleven bayts, 1,158 years after al-Shafi'i's death

Recited in Arabic. Translation mine.