Sunan an-Nasa'i 2134
سنن النسائي#2134Sahihأَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، عَنْ مُحَمَّدٍ، وَذَكَرَ، كَلِمَةً مَعْنَاهَا حَدَّثَنَا شُعْبَةُ، عَنْ سَلَمَةَ، قَالَ سَمِعْتُ أَبَا الْحَكَمِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الشَّهْرُ تِسْعٌ وَعِشْرُونَ يَوْمًا " .
It was narrated that Ibn 'Abbas said:"The Messenger of Allah said: 'The month is twenty-nine days
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiHasan
In-book reference: 22:45