Musnad Abu Ya'la al-Mawsili 159:125

Bishr ibn al-Walid narrated to us, Fulayh narrated to us, from Abdullah ibn Abd al-Rahman ibn Ma'mar, from Sa'id ibn Yasar, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever learns knowledge by which the Face of Allah is sought, but he does not learn it except to attain by it a portion of the world, he will not find the fragrance of Paradise on the Day of Resurrection."