Ja'far bin Muhammad (may Allah have mercy on him) narrates from his father, and he from his grandfather, that the Messenger of Allah (peace and blessings be upon him) forbade "cutting and gathering the harvest at night." Ja'far (may Allah have mercy on him) said: I think it was because of (giving to) the poor.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7511