The basis for this is that his statement,

“A resident may wipe for one day and one night, and a traveller for three days and their nights,”

explicitly permits wiping for this period. Its implication, however, does not apply universally. It is sufficient that what is left unstated should not have the same ruling as what is explicitly stated: a difference in even one case establishes this distinction. Thus, if wiping beyond this period is not permitted unconditionally, but is sometimes prohibited and sometimes permitted, the hadith has been duly applied. This is clear, and it is a very useful point.

Anyone who has undertaken journeys for Hajj, jihad, trade, or other purposes has seen that, on many occasions, removing the leather socks and performing ablution is impossible without harm exceeding what would suffice to permit tayammum. Consider, for comparison, someone whose permitted period expires while the enemy is facing him. The purpose of removing the socks is to wash the feet in ablution; wherever the obligation to wash the feet is waived, the obligation to remove the socks is also waived.

Ablution might be obligatory if the feet were exposed. When they are covered, however, he would need to remove the socks, wash his feet, and then put the socks on again, if his needs cannot otherwise be met. This differs from remaining as he is, since his state of purification remains intact, and from performing ablution and wiping over the socks, which may cause him no harm.

Thus, in these two situations, wiping is not subject to a time limit. Where the obligation of ablution is waived and he would turn to tayammum, continuing to wipe is preferable to tayammum. And where removing the socks and putting them on again would cause harm sufficient to permit tayammum, it is all the more fitting that it should permit wiping. Allah knows best.