Sayyidah Asma' bint Abi Bakr (may Allah be pleased with her) said: "In the time of the Messenger of Allah (peace and blessings be upon him), we used to pay Zakat al-Fitr with the same mudd (measure) with which we measured out our food."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20723