Allah, the most Sublime, the most Exalted, says, ‘(As for)...

Allah, the most Sublime, the most Exalted, says, ‘(As for) those of you who put away their wives by likening their backs to the backs of their mothers, these are not their mothers; their mothers are no other than those who gave birth to them, and most surely they utter a hateful word and a falsehood, and most surely Allah is Pardoning, Forgiving.

(As for) those who put away their wives by likening their backs to the backs of their mothers then recall what they said, they should free a captive before touching each other; to that you are admonished (to conform), and Allah is Aware of what you do. But whoever has no (such) means, let him fast for two months succesively before they touch each other’ (Holy Qur’an, 58-2-4).

Another (such type of obligatory fast) is fasting two successive months if a Muslim kills another Muslim by mistake and is unable to free a slave; this is obligatory on account of the verse saying: ‘And whoever kills a believer by mistake should free a believing slave, and blood-money should be paid to his people unless they remit it as alms; but if they be from a tribe hostile to you and he is a believer, the freeing of a believing slave (suffices), and if he is from a tribe between whom and you there is a covenant, the blood-money should be paid to his people along with the freeing of a believing slave; but if he cannot find (a slave), he should fast for two months successively a penance from Allah, and Allah is Knowing, Wise’ (Holy Qur’an, 4:92).

Another (such obligatory fast) is fasting three days as an act of atonement for breaking an oath if one is unable to feed the needy; Allah, the most Sublime, the most High, says, ‘Allah does not call you to account for what is vain of your oaths, but He calls you to account for making deliberate oaths; so, its expiation is the feeding of ten poor men out of the middling (food) whereby you feed your families, or their clothing, or the freeing of a slave; but whosoever cannot find (means), he, then, should fast for three days; this is the expiation of your oaths when you swear’ (Holy Qur’an, 5:89).

All types of such fast are consecutive, not a day now and a day then. "And the fast on account of causing an injury to the head while shaving it while performing the pilgrimage rites is also obligatory; Allah Almighty says, ‘Whoever among you is sick or has an ailment of the head, he (should effect) a compensation by fasting or offering alms or sacrificing’ (Holy Qur’an, 2:196).