Sunan an-Nasa'i 2130

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا أَبُو الأَحْوَصِ، عَنْ سِمَاكٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ لاَ تَصُومُوا قَبْلَ رَمَضَانَ صُومُوا لِلرُّؤْيَةِ وَأَفْطِرُوا لِلرُّؤْيَةِ فَإِنْ حَالَتْ دُونَهُ غَيَايَةٌ فَأَكْمِلُوا ثَلاَثِينَ ‏"‏ ‏.‏

It was narrated that Ibn 'Abbas said:"The Messenger of Allah said: "The Messenger of Allah said: Do not fast before Ramadan. Fast when you see it and stop fasting when you see it, and if clouds prevent you from seeing it, then complete Thirty (Days)

Reference

Sunan an-Nasa'i 2130

In-book

Book 22, Hadith 41