As far as Islamic Shari`ah is concerned, Tahajjud (optional late-night prayer) is not mandatory except on Prophet Muhammad (peace and blessings be upon him). But it is a highly recommended sunnah that every Muslim should be keen to observe it and gain its great reward.
Dr. Zulfiqar Ali Shah, member of the Executive Committee of the Fiqh Council of North America and the Religious Director of the Islamic Society of Milwaukee, states: Actually, Tahajjud is not mandatory except for Prophet Muhammad (peace and blessings be upon him), but it is a highly recommended sunnah. The Qur’an and Hadith emphasize a great deal upon worshipping Allah in the last third of the night. The Prophet (peace and blessings be upon him) used to take some rest before offering the Tahajjud so as to be fresh and be able to focus upon the Qur’anic recitation and the Tahajjud experience as a whole. He used to make the Witr Prayer as the last prayer of the night. That is what he recommended for his Ummah also.
It is permitted for a Muslim to recite from the Qur’an any number of surahs or ayahs in the Tahajjud. The Tahajjud could be 4, 6, 8, or 12 rak`ahs – whatever is possible for a person. Usually longer surahs, longer ayahs, longer ruku` and sujud are highly recommended. According to the Hadith reported by `A’ishah (may Allah be pleased with her), the Prophet (peace and blessings be upon him) used to perform 11 rak`ahs of qiyam in Ramadan as well as in other months, which means that he would perform eight rak`ahs of qiyam, two rak`ahs of shaf`, and one rak`ah of witr.