It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: If I did not think that the Muslims would fall into hardship, I would never stay behind from any expedition sent out in the way of Allah. But I do not have enough means to provide mounts for all of them, and I fear that after me they might start staying behind from participating in jihad.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 7344