Explanation

Hazrat Hamza bin Abdul Mattalib (R.A) was awarded with the title of “The lion of Allah”. His name was Hamzah ibn ‘Abdul-Muttalib ibn Haashim ibn ‘Abd-Manaaf ibn Qusayy ibn Kilaab ibn Murrah ibn Ka’b ibn Lu’ay ibn Ghaalib ibn Fihr ibn Maalik ibn An-Nadhr ibn Kinaanah. He was nicknamed ‘the Lion of Allaah’, and he was the uncle and foster brother of the Prophet, sallallaahu ‘alayhi wa sallam. According to the scholars may Allaah have mercy upon them it was Hamzah ibn ‘Abdul-Muttalib may Allaah be pleased with him who was nicknamed ‘the Lion of Allaah’. When the above name is mentioned, it refers to Hamzah may Allaah be pleased with him. However, there are many ‘lions of Allaah’, who fought for the sake of Allaah The Almighty, and there is no doubt that ‘Ali may Allaah be pleased with him was one of them. He was known for his bravery and strength, but the above expression (i.e. ‘the Lion of Allaah’) does not refer to him but to Hamzah may Allaah be pleased with him as we already mentioned. reference: islamweb.net