Is it permissible to perform ablution before the time [for prayer] begins? And is it obligatory to make the water reach the place of a head injury?

Entry of the time is not a condition for the validity of ablution, as evidenced by the action of the Prophet, peace and blessings be upon him, on the Day of Conquest, when he prayed several prayers with a single ablution. This implies that he performed some of them with an ablution he had performed before their time entered.

As for a head injury, it is not necessary to let water reach it during wiping, because those who stipulate wiping the entire head do not require water to penetrate it. Others have said that wiping part of the head is sufficient. The difference in opinion on this stems from the differing implications of the Arabic letter "ba" (ب). Those who consider it superfluous obligated wiping the entire head, while those who consider it to denote partiality obligated wiping only part of it. They cited the hadith (definition) of Al-Mughirah regarding the Prophet wiping over his forelock and turban.