“Whoever desires this world must seek knowledge, and whoever desires the Hereafter must seek knowledge.”

These are al-Shafi‘i’s words, not a hadith — a distinction worth keeping, since the sentence is often passed around as a saying of the Prophet (peace be upon him). Al-Bayhaqi records it with his chain to al-Rabi‘ ibn Sulayman, al-Shafi‘i’s student, who heard him say it, and an-Nawawi and Ibn Kathir quote it from him as well. The third clause often added today — “and whoever wants both” — is not in these reports.