"You see them bowing and prostrating [in prayer], seeking bounty from Allah and [His] pleasure. Their mark is on their faces from the trace of prostration..." Quran (48:29)

"Prostrate and draw near [to Allah]". (96:19)

The Prophet, peace and blessings be upon him, will recognise his Ummah on the day of Qiyamah by their bright faces due to performing Sajdah. (Ahmad, sahih)

“Allah has prohibited the Fire from devouring the marks of sujood. (Bukhari & Muslim)

Long Ruku and Sajdah of the Prophet :

"I happened to pray one night with the Prophet, peace and blessings be upon him. The Prophet started reciting al-Baqarah chapter of the Quran and I thought he would stop after 100 verses. But when he went beyond it I thought that he may want to recite the whole chapter in one Rakah. When he finished al-Baqarah I thought he would do Ruku but then he immediately started reciting al-Imran and when he finished he started reciting an-Nisa.
The Prophet was reciting very slowly with enough pauses and would do Tasbih (praising God) and Dua (supplication) according to the subject being discussed in the relevant Ayah.
After that the Prophet did Ruku (bowing). In Ruku he stayed as long as he did when he was in Qiyam (standing in prayer). After Ruku he stood up for almost same time and then he performed Sajdah (prostration) and stayed there as long as he recited Quran while doing Qiyam". (Hudaifa, may God be pleased with him, narrated this hadith in Sahih Muslim, Nasai)

Aisha RA relates in a hadith that because of the long Sajdah of the Prophet , she got apprehensive thinking that he had passed away. (Baihaqi)

One of the Prophet's companion, Abdullah ibn Zubayr RA, would pray with such concentration that when he was in Sajdah the sparrows would come flying and sit on his back. In a separate narration, Abdullah ibn Abbas , another companion, said if you want to see how the Prophet of Allah used to pray, you should copy how Abdullah ibn Zubayr used to pray.

Examples from the Sahaba, Tabaeen and the Salaf:

So, it is from the Sunnah to do long Ruku and Sujood in Nafil Salaah.

Below are mentioned some more examples of this from the Sahaba, Tabaeen and the Salaf:

Muslim Al-Makkee narrated: “Once I saw Ibn Az-Zubair RA go into Ruku so I read surah al-Baqarah, Aali Imraan, an-Nisaa’ and al-Maa’idah and he did not raise his head!” [Az-Zuhd of Abu Dawud, p. 348]

Abu Qatn narrated:
“I never saw Shu’bah making ruku except that I thought he forgot, nor have I ever seen him sitting between the two prostrations except that I thought he forgot.” [As-Siyaar, 7/207]

Ibn Wahb narrated:
Once I saw (Sufyan) Ath-Thawri in the Haram after maghrib prayer offering voluntary prayer. Then he went down into sujood (prostration) and he didn’t raise his head until the adhaan for ‘isha was made.” [As-Siyaar, 7/296]

‘Abdaan narrated:
“Hudbah Ibn Hakeen used to make tasbeeh in ruku and sujood more than thirty times.” [As-Siyaar,11/99]

May Allah give us the Tawfeeq for following these examples.Ameen.