Musnad Abu Ya'la al-Mawsili 114:14

Musa ibn Muhammad ibn Hayyan narrated to us, Sahl ibn Hammad narrated to us, Hafs ibn Sulayman narrated to us, Kathir ibn Shanzir narrated to us, from Ibn Sirin, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Seeking knowledge is an obligation upon every Muslim."