Question 448: Salaam alaykum. For salat al layl, when we recite the first eight rakah, at the beginning of every two rakah, do we have to recite niyah again? Also do we have to recite Tashud and Salaam like Fajr prayers, at the end of two rakah? So, how to make niyyat to pray salat al-Layl?
Answer 448: Niyyat (an obligatory act of prayer which is elemental (Rukn)), Tashahhud (to express testimony once in a prayer of two Rak’ats after raising the head from the last prostration, and twice in the prayers having three or four Rak’ats) and Salaam or Taslim (Salutation) are some of the obligatory acts relating to prayer whether it is mustahab or wajib prayer.
Out of the 11 Rak’ats of the night Nafilah, 8 Rak’ats should be offered with the niyyat of the Nafilah, 2 Rak’ats with the niyyat of Shaf’a, and 1 Rak’at with the Niyyat of Witr.
We are supposed to offer our prayers with the intention of Qurbat (a condition of wishing closeness to Allah, the Exalted and obedience to His Command). It is not obligatory to express it loudly, as it is something relating to the heart, as it is not obligatory to express it in the heart or speak in the imagination or present it in the mind, so that the person must only bring it into his thinking and the treasure of thought, f or example, that I am offering such and such prayer in obedience to His command.
Therefore we should make intention (to offer 8 Rak’ats with the niyyat of the Nafilah), recite Tashahhud and Salaam like Fajr prayer.
For further information in this regards, please refer to the following answers:
Index: Reciting Small Surah in Night Prayer, answer 573.
Index: Number of things which invalidate the prayer, answer 547.
Index: The Secret of Prayer / Sirr us-Salat, answer 589.
Index: Do we have to pray in Arabic language, answer 010.
Index: Saying Āmīn during prayer in validate the prayer, answer 017.
Index: Reciting Dua or Dikr in any language during the Prayer, answer 579.
 . Imam Khomeini (ra), Tahrir al-Wasilah, Vol. 1, Pg. 156-182; Tawzih al-Masael of maraja (with annotation of Imam Khomeini), Vol. 1, Pg. 600; The official website of the office of Sayyid Sistani, rules of Namaz.