It is permissible for a person to fast voluntarily for seven days or ten days one after another, because of the hadeeth of ‘Aa’ishah (may Allah be pleased with her) who said: The Messenger of Allah (blessings and peace of Allah be upon him) used to fast until we would say, he will never stop fasting, and he used not to fast until we would say, he will never fast. I never saw the Messenger of Allah (blessings and peace of Allah be upon him) fast any entire month except Ramadan, and I never saw him fast more than he did in Sha‘baan.
Narrated by al-Bukhaari (1833) and Muslim (1956).