The Prophet ﷺ said, "Fast when you sight the new moon, and break your fast when you sight it. If it is obscured from you, then complete the count as thirty. The month is like this, and like this, and like this," and he gestured with his fingers.
أن النبي ﷺ قال: صوموا الهلال- لرؤيته، وأفطروا لرؤيته، فإن غم عليكم فأكملوا العدة ثلاثين. والشهر هكذا وهكذا وهكذا وعقد.
This was narrated by Imam Ahmad (20432), Bazzar in Kashf Astar (970), and Abu Dawud Tayalisi (914), with Bayhaqi (4/206) reporting from him, all of them from the hadith of Imran Qattan, from Qatada, from Hasan, from Abu Bakra, as he related it. Bazzar said, "We do not know it from Abu Bakra except by this route, Imran narrated it alone." I say, Imran is Ibn Dawar Qattan, about whom there is disagreement, for Bukhari, Tirmidhi, Imam Ahmad, and others held a favorable opinion of him. So if we find nothing in his hadith that contradicts it, then it is hasan, and this is one such case given the abundance of its corroborating reports. Hasan here is the well-known Imam, and there is disagreement over whether he heard directly from Abu Bakra. Bahz bin Assad affirmed it while Daraqutni denied it, but the correct view is that he did hear from him, as is found in the Sahihayn, Sunan Nasai, and elsewhere.Similar in meaning is what was reported from Talq bin Ali, who said, the Messenger of Allah ﷺ said, "When you see the new moon, fast, and when you see it, break your fast, and if it is obscured from you, then complete the count." This was narrated by Imam Ahmad (16290) from Musa, who said, Muhammad bin Jabir narrated to us, from Qays bin Talq, from his father, who said (as he related it). Tabarani also narrated it in Al-Kabir (8/397) through two routes, Yahya bin Ishaq and Muhammad bin Sulayman Luwayn, both saying, Muhammad bin Jabir narrated to us with his chain, as he related it. He also narrated it from the hadith of Hisham bin Hassan, from Muhammad bin Jabir with his chain, similarly. Bayhaqi (4/208) narrated it through another route from Hisham bin Hassan, from Qays bin Talq, from his father, without mentioning "Muhammad bin Jabir" between them. One wonders, is there a gap here, or is this how it was found?Bukhari said, "Muhammad bin Jabir Abu Abdullah Suhaymi, from Hammad bin Abu Sulayman, and Qays bin Talq is not strong, they speak critically of him." Ibn Main said, "Muhammad bin Jabir is nothing." Nasai said, "Daif." They said, he is truthful, but his books were lost, so he came to rely on prompting, and was therefore weakened on that account.However, Tabarani (8/404) narrated it through another route from Muhammad bin Miskin Yamami, who said, Abdul-Rahman bin Awf bin Habban narrated to us, my father narrated to me, from Musa bin Umayr, from Qays bin Talq, from his father, and he related the hadith. In it is, "Until they see the new moon or the count is completed, then we do not stop fasting until they see it or the count is completed." Haythami said in Al-Majmu (3/14), "It contains someone I do not know."Similar in meaning is also what was reported from Umar bin Khattab, that the Messenger of Allah ﷺ said, "Do not advance this month, fast upon sighting it and break your fast upon sighting it, and if it is obscured from you, then count thirty." This was narrated by Fakihi in his Fawaid (53), from Abdul-Salam bin Asim Razi in Makkah, who said, Abu Zuhayr Abdul-Rahman bin Mughra narrated to us, Muhammad bin Ishaq narrated to us, from Muhammad bin Ibrahim, from Malik bin Abu Amir, from Umar bin Khattab, as he related it. Bayhaqi (4/207) narrated it by way of Fakihi. Tabarani also narrated it in Al-Awsat, in Majmu Bahrayn (1492), through another route from Abu Zuhayr, from Abdul-Rahman bin Mughra, with his chain, similarly. Tabarani said, "It is not narrated from Umar except by this chain, Abdul-Rahman narrated it alone."I say, Abdul-Rahman bin Mughra is disputed, but he is hasan in hadith apart from Amash, so his being alone in narrating it is not harmful. However, it contains Muhammad bin Ishaq, who practices tadlis, and I have not found an explicit statement from him confirming direct transmission. Scholars of knowledge do not accept him in matters of legal rulings when he does not state it explicitly.