Is it obligatory for someone who prays the two Rak'ahs of Sunnah after 'Isha' and then prays Witr with one Rak'ah to renew the intention daily for performing them, or is the intention of habitual practice sufficient?

The intention (niyyah (definition)) for performing an act of worship is the heart's awareness of the specific type of worship the body will undertake, such as the Fajr prayer or Witr. Its place is to be concurrent with the takbirat al-ihram (definition) (the opening Allahu Akbar) or immediately preceding it by a short, customary period. Habitually performing an act does not substitute for intention, nor does an intention made a long time in advance. Otherwise, the prayer would be considered a general supererogatory (nafl) prayer, due to the Prophet's (peace be upon him) saying: "Indeed, actions are by intentions, and indeed for every person is what he intended." So, whoever does not make a sound intention with its conditions, such as concurrence and specification, will not have that specific act (of worship) counted for him, but he will receive the reward of a general supererogatory prayer. Al-Nawawi and Ibn Qudamah clarified that specifying what is intended is a condition.