من لا يحضره الفقيه, volume 2, page 148

Hadith.1997 - Al-Halabi narrated from Abu Abdullah (as): "If a man has missed fasts from the Month of Ramadan, he may make them up in any month he wishes, in consecutive days. If he is unable to do so, he may make them up as he is able, counting the days. Whether he separates the days or fasts them consecutively, both are acceptable."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.

Hadith.1998 - Sulaiman ibn Ja'far Al-Ja'fari asked Abu Al-Hasan Al-Ridha (as) about a man who has missed days from the Month of Ramadan. Can he make them up non-consecutively? Imam (as) replied: "There is no harm in making up the fasts of the Month of Ramadan separately. The fasting that must not be separated is the fast of expiation for zhihar, expiation for shedding blood, and expiation for breaking an oath."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.

Hadith.1999 - Jameel narrated from Zurara from Abu Ja'far (as) regarding a man who becomes ill and the Month of Ramadan arrives while he is still sick. If he recovers after Month of Ramadan but does not make up the fasts before another Month of Ramadan arrives, what is his duty? Imam (as) replied: "He should give charity for the first missed Month of Ramadan (a mudd of food for each day) and fast the second Month of Ramadan. However, if he recovered in between the two Month of Ramadans and did not fast until the next Month of Ramadan came, then he must fast both Month of Ramadans and also give charity for the first."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.