Hadith 2510
Chapter 36: The Measure Of Zakatul-Fitr - كتاب الزكاة
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.
Reference : Sunan an-Nasa'i 2510
In-book reference : Book 23, Hadith 76