The Soft one appointed the tough one, and vice versa, in order to balance the affairs
In The Name of Allaah, The Most Merciful The Bestower of Mercy
Shaikh Saaleh Al-Fawzaan (may Allaah preserve him) stated:
Amr Ibnul Aas (radiyallaahu-anhu) was one of those given authority and so was Abu Ubaydah Ibn Al-Jarraah (radiyallaahu-anhu), and Khaalid Bin Al-Waleed (radiyallaahu-anhu) was given preference over them due to his bravery and usefulness in Jihaad.
When Abu Bakr (radiyallaahu-anhu) passed away, Umar Ibnul Khattaab (radiyallaahu-anhu) gave Abu Ubaydah authority over all of them-that is because Umar Ibnul khattaab (radiyallaahu-anhu) used to be stern for the sake of Allaah, so he appointed Abu Ubaydah for he was soft; and Abu Bakr was soft and Khaalid was stern against the disbelievers. Therefore, the soft one appointed the stern one and the stern one appointed the soft one in order to balance the affairs. These two (Abu Bakr and Umar) acted in a manner most beloved to Allaah with regards to fulfilling His Rights.
Indeed our Prophet (sallal-laahu-alayhi-wasalla)-the most perfect human being amongst the creation– was stern against the disbelievers and the hypocrites (i.e. those disbelievers who fought him), and Allaah favoured him with the most perfect revealed laws, as Allaah (The Most High) stated whilst describing his ummah: ”Muhammad is the Messenger of Allaah, and those who are with him are severe against disbelievers, and merciful among themselves’ 48:29
And He (The Most High) said about them: ”Humble towards the believers, stern towards the disbelievers, fighting in the Way of Allaah, and never afraid of the blame of the blamers’. 5:54
And it has been established in Bukhaari and Muslim that when the prophet (sallal-laahu-alayhi-wasallam) consulted his companions regarding the affair of the kuffaar prisoners of war at the battle of Badr, Abu Bakr suggested that they should be set free after taking ransom on their behalf and Umar suggested that they should be executed; so the Prophet (sallal-laahu-alayhi-wasallam) said: Indeed Allaah softens the hearts of men until they become softer than cloth (of linen or cotton), and He toughens the hearts of men until they become tougher than a rock. And your likeness O Abu Bakr is that of Ibraaheem (alayhis-salaam) when he said: ‘‘But whoever follows me, he verily is of me. And whoever disobeys me, – still You are indeed Oft-Forgiving, Most Merciful’. [14:36], and like Esaa (alayhis-salaam) when he said: ‘If You punish them, they are Your slaves, and if You forgive them, verily You, only You are the All Mighty, the All Wise.'[5:118]
And your likeness O Umar is that of Nuh (alayhis-salaam) when he said: ”And Nuh (Noah) said: ‘My Lord! Leave not one of the disbelievers on the earth!'[71:26] and like Musaa (alayhis-salaam) when he said: ‘Our Lord! You have indeed bestowed on Fir’aun (Pharaoh) and his chiefs splendour and wealth in the life of this world, our Lord! That they may lead men astray from Your Path. Our Lord! Destroy their wealth, and harden their hearts, so that they will not believe until they see the painful torment.'[10:88]
They (Abu Bakr and Umar) were as the Messenger described them during his lifetime, and they were (also) his disciples to the people of the earth.
[Source: Ad-waa Min Fataawaa Sheikhul Islaam Ibn Taymiyyah (rahimahullaah):Vol: 2, page: 568-569]