عَنْ أَسْمَاءَ بِنْتِ أَبِي بَكْرٍ رَضِيَ اللَّهُ عَنْهُمَا قَالَتْ: كُنَّا نُؤَدِّي زَكَاةَ الْفِطْرِ عَلَى عَهْدِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ مُدَّيْنِ مِنْ قَمْحٍ بِالْمُدِّ الَّذِي تَقْتَاتُونَ بِهِ
It is narrated from Sayyida Asma bint Abi Bakr (may Allah be pleased with her), she says: In the time of the Messenger of Allah (peace and blessings be upon him), we used to give two mudd of wheat as Sadaqat al-Fitr on behalf of a person. This is the same mudd with which you measure grain for transactions.