It is narrated from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "By the One in whose hand is the soul of Muhammad (peace and blessings be upon him)! Were it not that I am mindful of hardship upon the people, I would not stay behind any expedition (sariyyah) that fights in the path of Allah; but I cannot arrange mounts for them, nor do they have the means to follow me, nor are they content to remain behind me."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18486