Fiqh-us Sunnah
Fiqh-us-Sunnah Volume 002, Supererogatory Prayer, Fiqh 2.119.
Hadith Section/Book: Fiqh-us Sunnah Fiqh-us Sunnah Section 37. Combining two Prayers
Section : Validity of combined prayers after their legal excuse ceases to exist.
In al-Mughni it is stated: "If someone performs both prayers at the time of the earlier salah and then his reason for doing so ceases to exist after he has completed the salah and before the time of the next salah begins [i.e., the next salah being the one which he had just prayed during the earlier time], then what he has done is sufficient for him and he need not repeat the second salah at its proper time. Since he performed the salah in a proper manner, he is free from any extra obligation due to that action. He fulfilled his obligation during a circumstance in which he had some legal excuse, and his action is not invalidated by the fact that this excuse no longer exists. This is similar to the case of a person who perform tayammum, and after he finishes his salah, he finds water."
In al-Mughni it is stated: "If someone performs both prayers at the time of the earlier salah and then his reason for doing so ceases to exist after he has completed the salah and before the time of the next salah begins [i.e., the next salah being the one which he had just prayed during the earlier time], then what he has done is sufficient for him and he need not repeat the second salah at its proper time. Since he performed the salah in a proper manner, he is free from any extra obligation due to that action. He fulfilled his obligation during a circumstance in which he had some legal excuse, and his action is not invalidated by the fact that this excuse no longer exists. This is similar to the case of a person who perform tayammum, and after he finishes his salah, he finds water."