Sadaqat al-Fitr is an obligatory duty

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) ordered a caller in the middle of Makkah to announce: "Indeed, Sadaqat al-Fitr is an obligatory right upon every Muslim, whether young or old, male or female, free or slave, town-dweller or desert-dweller, and it is one sa' of barley or one sa' of dates."
The chain of this hadith is sahih, but they did not record it with these words.

It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) encouraged the charity of Ramadan (Sadaqat al-Fitr): that one sa' of dates, or one sa' of barley, or one sa' of wheat be paid on behalf of every person.
This hadith is sahih, and it also has a sahih supporting narration (shahid).

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) made Sadaqat al-Fitr obligatory, as one sa' of dates or one sa' of wheat, on every free person, slave, male and female among the Muslims.

When Sadaqat al-Fitr was mentioned in the presence of Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him), he said: "I will give (as Sadaqat al-Fitr) only what I used to give in the blessed time of the Messenger of Allah (peace and blessings be upon him): one sa' of dates, or one sa' of wheat, or one sa' of barley, or one sa' of dried cheese (aqit)." A man from the gathering said to him: "Or two mudd (of wheat)?" He replied: "No, that is the valuation set by Sayyiduna Mu'awiyah (may Allah be pleased with him); I will neither accept it nor act upon it."
The chains that I have presented earlier regarding the sa' of wheat are all sahih, and the most famous of them is the hadith of Abu Ma'shar, which he narrated from Nafi' from Sayyiduna Ibn Umar (may Allah be pleased with them both), for which we obtained a high chain; but I did not record it here because it was not on the condition of this book (al-Mustadrak).

It is narrated from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said regarding Sadaqat al-Fitr: "On behalf of every young and old, free and slave, (let there be given) one sa' of wheat or one sa' of dates."
Thus this narration has been reported from Sayyiduna Ali (may Allah be pleased with him) as marfu' (as a saying of the Prophet, peace and blessings be upon him), whereas other narrators have reported it as mawquf (as Sayyiduna Ali's own statement).

It is narrated from al-Harith that he heard Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) ordering Sadaqat al-Fitr, saying: "One sa' of dates, or one sa' of barley, or one sa' of wheat, or (hulless) barley, or one sa' of raisins (give these as Sadaqat al-Fitr)."

Sayyiduna Zayd bin Thabit (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) addressed us in a sermon and said: "Whoever has food should give in charity one sa' of wheat, or one sa' of barley, or one sa' of dates, or one sa' of flour, or one sa' of raisins, or one sa' of (hulless) barley."

Sayyidah Asma' bint Abi Bakr (may Allah be pleased with them both) narrates that in the time of the Messenger of Allah (peace and blessings be upon him) they used to give Sadaqat al-Fitr with the mudd (measure) with which the household measured its food, or with the sa' with which they measured their grain, and all the people of Madinah used to do this.
This hadith is sahih on the condition of the two Shaykhs, but they did not record it; and this narration is the main proof in the debate between Imam Malik and Imam Abu Yusuf (on the quantity of the sa').