Sayyiduna Uqbah bin Amir (may Allah be pleased with him) and Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) state: The Prophet (peace and blessings be upon him) would be in a state of major ritual impurity (janabah) at true dawn, then he would take a ritual bath (ghusl) and keep the fast.
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains a group of narrators whom I have not found anyone mentioning.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4834
Hadith Gradingمحدثین:إسناده ضعیف
Hadith Takhrij«اخرجه الطبراني فى المعجم الكبير (326/17 و315/18)»