Is intention (niyyah) obligatory for prayer, and should the prayer be repeated if it was forgotten?

Scholars have unanimously agreed on the obligation of intention (niyyah) for prayer, and that it is not valid without it. There is no harm in preceding the takbīrat al-iḥrām with the intention by a short time, though aligning it with the takbīr is better. If a worshipper forgets to make the intention for prayer, then it is not valid and must be repeated. Intention is a simple matter, and it is difficult to imagine a sane adult forgetting it, as his ablution and standing for prayer are evidence of his intention to perform it. "Intention follows knowledge; whoever knows what he intends to do has indeed intended it, for it cannot be done without intention."