Imam Aḥmad ibn Naqīb al-Masri says in ʿUmdat as-Sālik:
'It is recommended to fast six days from Shawwāl, and it is recommended to do them consecutively after Eid. It one splits them up, it is permissible.'
Imam Muṣṭafā al-Bughā says in his commentary:
'It is on the authority of Abū Ayūb, may Allah be pleased with him, that the Messenger of Allah, may Allah be pleased with him, said, {Whoever fasts Ramaḍān and follows it up with six days from Shawwāl, it's as if he has fasted his entire life[1].}
This is because one good deed is written down as ten, and thus Ramadan is like ten months and six days is like two months, completing an entire year. Therefore, if one does this every year, it's as if one fasts one's entire life.'
(The ḥadīth is narrated by Muslim in the Book of Fasting in the chapter on the recommendation to fast six days from the month of Shawwāl, following Ramaḍān, no. 1164; by Abū Dāwūd in the Book of Fasting in the chapter on fasting six days from Shawwāl, no. 2433; by at-Tirmidhī in the Book of Fasting in the chapter on what has reached us regarding six days from Shawwāl, no. 759, and he said it is ḥasan ṣaḥīḥ; by an-Nasāʾī in al-Kubrā in the Book of Fasting in the chapter mentioning the transmitters of Abū Ayūb's report and how they differ therein, after the chapter on fasting six days from Shawwāl, no. 2862-2867; and by Ibn Mājah in the Book of Fasting in the chapter on fasting six days from Shawwāl, no. 1716.)
[Translated from Tanwīr al-Masālik bi Sharḥ wa Adillah ʿUmdat as-Sālik wa ʿUddat an-Nāsik by Imam Musṭafā Dīb al-Bughā (Damascus: Dār al-Muṣṭafā, 1431/2010), v.1, p.532]
[1] Ar. ad-dahr, which in the normal context of the ḥadīth means an entire year, but Imam Muṣṭafā al-Bughā will explain how it could mean an entire lifetime.
