Hadith 690
Chapter 6: What Has Been Related About: The Month Is Twenty-Nine Days - كتاب الصوم عن رسول الله صلى الله عليه وسلم
حَدَّثَنَا عَلِيُّ بْنُ حُجْرٍ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ، أَنَّهُ قَالَ آلَى رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْ نِسَائِهِ شَهْرًا فَأَقَامَ فِي مَشْرُبَةٍ تِسْعًا وَعِشْرِينَ يَوْمًا قَالُوا يَا رَسُولَ اللَّهِ إِنَّكَ آلَيْتَ شَهْرًا فَقَالَ " الشَّهْرُ تِسْعٌ وَعِشْرُونَ " . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ .
Anas narrated:
"The Messenger of Allah vowed to stay away from his wives for a month, so he stayed in a loft for twenty-nine days. They said: 'O Messenger of Allah, your vow was for a month,' so he said: 'The month is twenty-nine (days).'"
Reference : Jami` at-Tirmidhi 690
In-book reference : Book 8, Hadith 9