ENOfficial
It was narrated that Ibn 'Abbas said: "The Messenger of Allah (ﷺ) said: "The Messenger of Allah (ﷺ) 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)."'
Source: sunnah.com
Other translations
Arabic
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 " .
sunnah.com