The Messenger of Allah ﷺ said, "When you see the crescent moon, begin fasting, and when you see it again, break your fast. If it is obscured from you, then fast thirty days."
حدثنا عبد الرزاق حدثنا معمر عن الزهري عن ابن المسيب وأبي سلمة أو عن أحدهما
قال رسول الله ﷺ إذا رأيتم الهلال فصوموا وإذا رأيتموه فأفطروا فإن غم عليكم فصوموا ثلاثين يوما
Its isnad is sahih. The doubt as to whether it is from Saeed bin Musayab and Abu Salama together, or from one of them, does not affect its soundness, since it is only an uncertainty between two trustworthy, authoritative narrators. It appears that the doubt here originates from Abdul-Razzaq, for the hadith is established through the narration of both of them. Hadith 7507 has already passed by the route of Abdul-Ala from Maamar, from Zuhri, from Abu Salama alone, without any doubt, and 7071 has already passed by the route of Ibrahim bin Saad from Zuhri, from Ibn Musayab alone.