Shulchan Aruch EH 77:2,3. Rashbo teshuva 7:414 and accepted by the Radvaz, Beis Yosef EH 154. GRA 5.
The Chazon Ish Gittin 99:2 says that if the husband does not have a Torah obligation to divorce and the beis din instructs him to divorce, without applying any coercion other than their decree, their ruling itself creates its own oness/coercion and the Get is invalid as the beis din caused the husband to think he had a Torah obligation to divorce, whereas he did not, and thus he gave the Get mistakenly.