حَدَّثَنَا مُعَاوِيَةُ بْنُ عَمْرٍو حَدَّثَنَا زَائِدَةُ عَنْ سِمَاكِ بْنِ حَرْبٍ عَنْ عِكْرِمَةَ عَنِ ابْنِ عَبَّاسٍ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ حَالَ دُونَهُ غَيَايَةٌ فَأَكْمِلُوا الْعِدَّةَ وَالشَّهْرُ تِسْعٌ وَعِشْرُونَ يَعْنِي أَنَّهُ نَاقِصٌ
مسند امام احمد بن حنبل · Hadith 2335
It was narrated that Ibn 'Abbas (RA) said: The Messenger of Allah (PBUH) said: "Fast when you Musnad of 'Abdullah bin al-'Abbas (RA) see il [the new moon] and break the fast when you see it, and if clouds prevent you from seeing it, then complete the number [of days, in the month]; the month is twenty-nine days," meaning that it may be shorter [than thirty days].
Translated by Dr. Muhammad Muhsin Khan · Reviewed by Islamic University of Madinah · Published March 2020