Muhammad bin al-Munkadir (may Allah have mercy on him) said: It reached me that the Messenger of Allah (peace and blessings be upon him) was asked about making up the fasts of Ramadan separately, and he (peace and blessings be upon him) said: "That is up to you. Tell me, if someone owes a debt and he pays one or two dirhams of it, would that not be payment? So Allah has a greater right to overlook and forgive."
Ali (al-Daraqutni) said: Its chain is hasan (good) except that it is mursal; others besides Abu Bakr narrated it from Yahya bin Sulaym as connected, but it is not established as connected. It has also been narrated marfu' from Ibn Umar through another, weak, route; and in contrast to it, a marfu' report from Abu Hurayrah forbidding splitting has been narrated, but how could that be authentic when the view of Abu Hurayrah was that splitting is permissible and the view of Ibn Umar was to fast consecutively? It has also been narrated marfu' from Abdullah bin Amr bin al-As through another route on the permissibility of splitting, and none of that is authentic.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8243