The Prophet ﷺ said, "Jibreel came to me and said, 'The month is twenty-nine days.'"
عن النبي ﷺ قال: أتاني جبريل عليه السلام، فقال: الشهر تسع وعشرون يوما.
Nasai (2133) and Ahmad (1885) both narrated it from the hadith of Shuba, from Salama bin Kuhayl, from Abu Hakam, from Ibn Abbas, and he related it. Abu Hakam's name was Imran bin Harith Sulami, and its chain of transmission is sahih.In another narration from him, he said, "The Messenger of Allah ﷺ kept away from his wives for a month. When twenty-nine days had passed, Jibreel came to him and said, 'Your oath has been fulfilled, and the month is complete.'"Ahmad (2103) narrated it from Amr bin Muhammad Abu Saeed Anqazi, who said, Sufyan informed us, from Salama bin Kuhayl, from Imran, from Ibn Abbas, and he related it. Its chain of transmission is sahih, and Imran is the same Ibn Harith Sulami, Abu Hakam, mentioned earlier.