The Book of Fasting
وَحَدَّثَنَا عَبْدُ الْوَارِثِ بْنُ عَبْدِ الصَّمَدِ، حَدَّثَنِي أَبِي، عَنْ جَدِّي، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، - رضى الله عنهما - عَنِ النَّبِيِّ صلى الله عليه وسلم بِمِثْلِهِ وَلَمْ يَقُلْ فِي رَمَضَانَ .A hadith like this has been transmitted by Ibn 'Umar (Allah be pleased with both of them), but he did not make mention of (the words):" During the month of Ramadan
From Sahih Muslim, whose reports the scholars accept as authentic.