“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). An-Nawawi quotes it from him in Tahdhib al-Asma’ wa’l-Lughat, and Ibn Kathir in his Tabaqat of the Shafi‘i scholars. The version they preserve has a third clause: and whoever wants both must seek knowledge.