I believe it is reported as a marfu hadith, "Whoever forgets a prayer should perform it when he remembers it, and again the following day at its proper time."
حدثنا عفان حدثنا همام أخبرنا بشر بن حرب
أحسبه مرفوعا من نسي صلاة فليصلها حين يذكرها ومن الغد للوقت
Its chain of transmission is hasan, due to Bishr bin Harb, about whom there is some discussion, but he is truthful and his hadith is accepted. This hadith was already mentioned at 13484, 13756, and 13942.