أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا حَمَّادٌ، عَنْ أَيُّوبَ، عَنْ أَبِي رَجَاءٍ، قَالَ سَمِعْتُ ابْنَ عَبَّاسٍ، يَخْطُبُ عَلَى مِنْبَرِكُمْ – يَعْنِي مِنْبَرَ الْبَصْرَةِ – يَقُولُ صَدَقَةُ الْفِطْرِ صَاعٌ مِنْ طَعَامٍ ‏.‏ قَالَ أَبُو عَبْدِ الرَّحْمَنِ هَذَا أَثْبَتُ الثَّلاَثَةِ ‏.‏

It was narrated that Abu Raja' said: I heard Ibn 'Abbas deliver a Khutbah from your Minbar – meaning the Minbar in Al-Basrah – saying: 'Sadaqatul Fitr is a Sa' of food." (Sahih)
Abu 'Abdur-Rahman (An-Nasa'i) said: This is the most reliable of the three.