The Messenger of Allah ﷺ said, "Fast when you see it and break your fast when you see it. If clouds obscure it, complete the count. The month is twenty-nine days," meaning that it is short.
حدثنا معاوية بن عمرو حدثنا زائدة عن سماك بن حرب عن عكرمة
قال رسول الله ﷺ صوموا لرؤيته وأفطروا لرؤيته فإن حال دونه غياية فأكملوا العدة والشهر تسع وعشرون يعني أنه ناقص
Its chain of transmission is sahih, and it is a repetition of 1985. Al-ghiyayah, with two consecutive y-sounds, means anything that shades a person from above his head, like a cloud or similar. In H it appears as ghiyabah with a b, but we have kept what appears in K, which is correct. The commentator on Tirmidhi (2:34) quoted from Ayni, who said, "This is the well-known reading of this hadith." Ibn Arabi said, "It is permissible to replace the final y with a b, deriving it from ghayb, meaning what is hidden and concealed from you."