Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Whoever fights in the path of Allah even for the length of a camel's fuwaq, Paradise becomes binding for him." And the 'fuwaq' of a she-camel is the time it takes her to let down her milk for the one milking her.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17339