Is it obligatory (wajib) to recite “Allahu Akbar” (God is the greatest) before bowing (ruku?), or when rising from prostration (sajdah), or for any acts other than takbiratul ihram?
“Allahu Akbar” before/after ruku’ and sajdah are not obligatory (wajib). Only takbiratul ihram is obligatory (wajib).
Please refer to:
957. To say Allahu Akbar in the beginning of every prayer is obligatory, and one of its pillars (Rukns), and it is necessary that every letter and the two words are uttered in proper succession. It is also necessary that these two words should be pronounced in correct Arabic. If a person pronounces these words incorrectly, or utters their translation, it will not be valid.
https://www.sistani.org/english/book/48/2219/
https://www.sistani.org/english/book/48/2222/
https://www.sistani.org/english/book/48/2225/