Nafi' (may Allah have mercy on him) narrates from Abdullah bin Umar (may Allah be pleased with them both) that he would not make up Ramadan separately. This is what Ibn Umar (may Allah be pleased with them both) said, but there is disagreement about it from Ali bin Abi Talib (may Allah be pleased with him).
Its narrator is al-Harith al-A'war, and al-Harith is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8247