Sayyiduna Sufyan bin Atiyyah bin Rabi'ah al-Thaqafi (may Allah be pleased with him) narrates: Our delegation from Thaqif came to the Messenger of Allah (peace and blessings be upon him), and they accepted Islam in the middle of Ramadan. So the Messenger of Allah (peace and blessings be upon him) ordered them to fast the rest of Ramadan with him, and he did not order them to make up what they had missed.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6371