Nafi' (may Allah have mercy on him) says that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: "The Messenger of Allah (peace and blessings be upon him) prescribed sadaqat al-fitr as a sa' of dates or of wheat, upon every free person and slave, Muslim man and woman."
This is how Sa'id bin Abd al-Rahman al-Jumahi narrated it, and the mention of wheat in it is not preserved (mahfuz).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7703