It is reported from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The best charity is lending a milch animal: a Muslim man who lends a household a she-camel that goes out in the morning with a vessel (of milk) and returns in the evening with two vessels. Its reward with Allah is immense."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7801