Riyad-us-Saliheen 1385
Anas (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "He who goes forth in search of knowledge is considered as struggling in the Cause of Allah until he returns."
[At-Tirmidhi]
In this Hadith the acquisition of knowledge has been equated with Jihad for the sake of Allah. Sheikh Al-Albani has regarded this Hadith weak in his "Takhriju Fiqh As-Sunnah".