  In this article, we derive a sub convexity estimate of Hecke eigen cusp forms
associated to certain cocompact arithmetic subgroups of SL(2,R). The main
result can be considered as the holomorphic version of the estimate of Hecke
eigen Maass forms, derived in a famous paper of Iwaniec and Sarnak. A stronger
estimate was derived by Khayutin and Steiner in arXiv:2009.07194. However,
techniques used in both the papers are very different.
