Sunan an-Nasai 17031
In-book no 2295أَخْبَرَنَا إِسْمَاعِيلُ بْنُ مَسْعُودٍ، قَالَ حَدَّثَنَا خَالِدٌ، عَنْ شُعْبَةَ، عَنْ مَنْصُورٍ، عَنْ مُجَاهِدٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ خَرَجَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِلَى مَكَّةَ فَصَامَ حَتَّى أَتَى عُسْفَانَ فَدَعَا بِقَدَحٍ فَشَرِبَ - قَالَ شُعْبَةُ - فِي رَمَضَانَ فَكَانَ ابْنُ عَبَّاسٍ يَقُولُ مَنْ شَاءَ صَامَ وَمَنْ شَاءَ أَفْطَرَ .
It was narrated that Ibn 'Abbas said: "The Messenger of Allah went out to Makkah, and he fasted until he came to 'Usfan. Then he called for a cup and drank." (One of the narrators) Shubah said: "(That was) in Ramadan. Ibn 'Abbas used to say: 'Whoever wants to fast, may fast, and whoever wants to break may break his fast."'