Hadith 4:424
Abu Bakr ibn Ayyash narrated to us, from Layth, from Mujahid, who said: If a person has makeup days (qada) from Ramadan [to perform], and he performs voluntary (tatawwu') fasts, then it counts as the makeup [fast], even if he did not intend it as such.
حدثنا أبو بكر بن عياش عن ليث عن مجاهد قال إذا كان علي الرجل قضاء من رمضان فتطوع فهو قضاء وإن لم يرده