Art. 33 No Contracting State shall expel or return (“refouler”) a refugee in any manner whatsoever to the frontiers of territories where his life or freedom would be threatened on account of his race, religion, nationality, membership of a particular social group or political opinion.
1951 Refugee Convention
Art. 3 (1) [n]o State Party shall expel, return (‘refouler’) or extradite a person to another State where there are substantial grounds for believing that he would be in danger of being subjected to torture.
Art. 3 (2) [f]or the purpose of determining whether there are such grounds, the competent authorities shall take into account all relevant considerations including, where applicable, the existence in the State concerned of a consistent pattern of gross, flagrant or mass violations of human rights.
UN Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment
European Convention on Human Rights article 3 provides absolute protection against torture and inhuman or degrading treatment or punishment.
Human Rights and the Refugee Definition : comparative legal practice and theory by 
Refuge from Inhumanity? War Refugees and International Humanitarian Law by