Salim (ra) narrated from his father, I narrated this dream (to my sister Umm al-Muminin) to Hafsa (ra). Then Hafsa (RA) narrated it to the Messenger of Allah (SAW). Then he said, ‘Abdullah is such a good man! If he woke up at night, he would perform the prayers (tahajjud)! ' (Bukhari, Hadith: 1122)
Urwah (may Allah be pleased with him) said: Ayesha (may Allah be pleased with her) informed me that the Messenger of Allah (may peace be upon him) used to perform 11 rak'ats of prayers and that was his (normal) prayers. In that prayer he used to do one sijdah so much that one of you (from sijdah) could recite fifty verses before raising his head. And he used to perform two rak'ats of prayers before the Fajr (obligatory) prayers. Then he would lie on his right side until the muezzin came to him for prayers. (Bukhari, Hadith: 1123)
Sitting in naafal prayers
Imran ibn Husain (may Allah be pleased with him) said: He was a patient of hemorrhoids. He said: I asked the Messenger of Allah (may peace be upon him) about the person who sat and prayed. He said that the one who prays standing is better and the one who prays while sitting is half the reward of the one who stands and the one who prays while lying is half the reward of the one who prays sitting. (Bukhari, Hadith: 1117)
Umm al-Muminin Ayesha (may Allah be pleased with her) said that she had never seen the Messenger of Allah (may peace be upon him) perform night prayers before reaching old age. After (old age) he used to sit and recite Kirat. When he wished to bow, he would stand up and recite about 30 or 40 verses. (Bukhari, Hadith: 1117)
If unable to perform standing prayers
Imran Ibn Husain (RA) said, I had hemorrhoids. So I asked about the prayers in the service of the Messenger of Allah (peace be upon him). He said, "Stand up and perform the prayers. If you can't do that, sit down. If you can't do that, then lie down." (Bukhari, Hadith: 1117)
It is better to recite Tahajjud in that part of the night
Abdullah ibn Amr ibn al-As (may Allah be pleased with him) said that the Messenger of Allah (may peace be upon him) told him that the most beloved prayer to Allah is the prayer of Dawood (peace be upon him). And the most beloved fast to Allah is the fast of Dawood (AS). He [David (pbuh)] slept until midnight, performed one-third of the tahajjud prayers, and slept one-sixth of the night. He used to fast one day and fast one day. (Bukhari, Hadith: 1131)
tahajjud is the biggest part of our salat and every muslim have to do tahajjud salat