I know of no place where a classical scholar calls Islam a "religion of peace".
Firstly because the term "religion of peace" is new, and has come into popular use after the September 11 Attacks, and before that had been used for Christianity in the 19th century. It wasn't really a term used in the time of the classical scholars.
In 1996, the Organisation of The Islamic Conference agreed to call for "pragmatic and constructive steps to counter the negative propaganda against Islam; to remove and rectify misunderstandings; and to present the true image of Islam: the religion of peace and tolerance." (Wikipedia)
Probably one of the first times there is a documented use of "religion of peace" to describe Islam.
Secondly because Islam is not a "religion of peace". It is as you said yourself better described as the religion of mercy and justice. And often times the path of mercy and justice is not the path of peace. e.g. Islam would allow or even encourage a Khalifa to take military action (Jihad) fighting against Hitler in WW2 even though it is not "peaceful".
Fighting for self defense:
Fight in the way of Allah those who fight you, but do not transgress. Indeed Allah does not like transgressors. (2:190)
Fighting for victims of oppression:
What is it with you that you do not fight in the way of Allah and the degraded men, women, and children, who say, ‘Our Lord, bring us out of this town whose people are wrongdoers, and appoint for us a guardian from You, and appoint for us a helper from You’?