(9) باب إِكْمَالِ شَعْبَانَ ثَلاَثِينَ إِذَا كَانَ غَيْمٌ وَذِكْرِ اخْتِلاَفِ النَّاقِلِينَ عَنْ أَبِي هُرَيْرَةَ
It was narrated that Abu Hurairah said:
"the Messenger of Allah said: 'Fast when you see it and stop fasting when you see it, and if it is obscured from you (too cloudy), then count it as thirty (days). "'
Reference: Sunan an-Nasa'i 2117
In-book reference: Book 22, Hadith 28
English translation: Vol. 3, Book 22, Hadith 2119