Sufyan bin Atiyyah (may Allah have mercy on him) narrates that a delegation from Thaqif came to the Messenger of Allah (peace and blessings be upon him). A tent was pitched for them, and they accepted Islam in the middle of Ramadan. The Messenger of Allah (peace and blessings be upon him) gave them the command, so they fasted the days that remained ahead, and he (peace and blessings be upon him) did not order them to make up the days they had missed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8308