Is it permissible to fast the six days of Shawwal immediately after Eid al-Fitr, or after three days have passed since Eid?

It is recommended to fast six days of Shawwal after Ramadan, based on the Prophet, peace and blessings be upon him, who said: "Whoever fasts Ramadan and then follows it with six days of Shawwal, it is as if he has fasted the entire year." It is preferable to fast them consecutively immediately after Eid al-Fitr, but if they are fasted separately or delayed until the end of Shawwal, the virtue is still attained.