Muhammad bin Ahmad bin al-Bara' (may Allah have mercy on him) says: I heard Ali bin al-Madini (may Allah have mercy on him) when he was asked about the hadith of Ibn Abbas (may Allah be pleased with them both) from the Prophet (peace and blessings be upon him) regarding sadaqat al-fitr, and he said: It is a Basran hadith, and its chain is mursal.
Ali also said: Al-Hasan did not hear from Ibn Abbas and never saw him at all; he was in Madinah during the days when Ibn Abbas was (governor) over Basrah. And Ali said to me regarding al-Hasan's words "Ibn Abbas addressed us in Basrah": This is just like Thabit's saying "Imran bin Husayn came to us," Mujahid's saying "Ali came out to us," and al-Hasan's saying "Suraqah bin Malik bin Ju'shum narrated to them"; al-Hasan did not hear from Ibn Abbas.
Shaykh Abu Ahmad said: The hadith of al-Hasan from Ibn Abbas is mursal. And we have narrated from Abu Raja' al-Utaridi, who heard it from Ibn Abbas in this same sermon, that in sadaqat al-fitr it is a sa' of food.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7713