Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: "By the One in whose hand is my soul, were it not that it would be hard on the believers, I would not stay behind any fighting that is fought in His path; but I do not find enough means to mount them all, nor do they have enough means to follow me, and their hearts are not content to stay behind after I have gone out to fight."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17822