Is it necessary to recite Bismillah before reciting Qur'an if I am starting recitation from a portion of Qur'an which is not start of a Surah? Kindly answer with some proof from Hadith/Qur'an?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If the reader starts the recitation of the Qur'an but not at the beginning of the Surah, then he has a choice to say “Bismi Allah” or not, as the scholars of recitations stated.

Imam Al-Shatibi (may Allah be pleased with him) said: ‘One must say ‘Bismi Allah’ at the beginning of the Surah, but if one reads from other than the beginning of the Surah, or inside the chapters of the Surah, then he has the choice whether to say “Bismi Allah” or not.’

Ibn Barri said in Ad-Durar Al-Lawami’: ‘Some reciters have chosen to recite it at the beginning of the parts of the chapter ‘Juz’ because of its excellence.’  What is meant by Juz’ (part of the chapter) is Hizb (one sixtieth of the whole Qur'an ), or Thumn one eighth of the Hizb and so on.

Allah knows best.