اللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ لَا إِلَهَ إِلَّا اللَّهُ وَاللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ وَلِلَّهِ الْحَمْدُ
Recitation of Takbeer al-Tashreeq is waajib after every Fardh Salaah commencing from the Fajr of the 9th of Dhul-Hijjah until after the Asr of the 13th of Dhul-Hijjah. (Shami vol. 1 pg. 406)
If one intentionally leaves out the Takbeer al-Tashreeq he/she will be sinful on account of omitting a Waajib of Deen.
It should be recited immediately after every Fardh salaah (not after Witr or Nafl), whether the salaah was performed individually or in congregation (Jamaat). A person who joins the Jamaat late in the Masjid will say the Takbeer after he has performed his Salah. He will also say it aloud.
If the person remembers whilst seated on the musallah and has not yet spoken to anyone or engaged in any worldly activity, then he should recite the Takbeer al-Tashreeq. However, if he spoke or engaged in any worldly activity, then Takbeer al-Tashreeq will fall off. (Durre Mukhtaar)