Narrated Ubayy ibn Kab: The Prophet ﷺ used to observe Itikaf during the last ten days of Ramadan. One year he did not observe Itikaf. When the next year came, he observed Itikaf for twenty nights (i. e. days).
Related hadith on this topic
Explanation & Benefits
Shaykh Umar Farooq Saeedi
Benefits and Issues:
Making up missed voluntary acts of worship (nafl a‘maal) is not obligatory, but there is great reward and virtue in performing them.
In particular, the Prophet sallallahu alayhi wa sallam used to be diligent in this regard.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2463
Maulana Ataullah Sajid
Benefit:
If the incident mentioned in this hadith is the same as the one mentioned in the previous hadith, then "the following year" refers to the year after next, skipping one year. This is because the Ramadan in which there was travel was during the conquest of Makkah in 8 AH, and the Prophet (sallallahu alayhi wa sallam) performed i'tikaf for twenty days in the Ramadan of 10 AH. It is possible that he also performed i'tikaf for twenty days in 9 AH.
And Allah knows best.
Source: Commentary on Sunan Ibn Mājah by Mawlānā ‘Atā’ullāh Sājid, Page: 1770