Ibn Jurayj said: Mujahid (may Allah have mercy on him) used to say that whoever among you (hunts) deliberately kills (the game) without forgetting, while in the state of ihram, and he has no other intention besides this, then he has made a mistake and it has become lawful, and there is no concession for him; and whoever kills it without remembering its sanctity or had some other intention besides this, then he has also made a mistake. This (mistaken) intention is such that for it, there is expiation with an animal of its like.
Hadith Referenceمسند الإمام الشافعي / كتاب الحج / 880
Hadith Takhrijاسناده حسن اخرجه البيهقى فى المعرفة السنن والآثار (15293)۔