Imam al-Shafi'i (may Allah have mercy on him) said: "Allah, the Blessed and Exalted, says: 'Fight those of the disbelievers who are near you.' [Surah al-Tawbah: 123]"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / Q17881
Ibn Ishaq (may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) intended to fight. He (peace and blessings be upon him) began jihad against the enemies whom Allah Almighty had commanded him (to fight), and Allah Almighty had commanded him to fight the nearest polytheists first.
Imam al-Shafi'i (may Allah have mercy on him) said: If the condition of the enemy differs, that is, some of them cause more harm than others or are more feared than others, then the commander will fight first the enemy from whom there is more danger, even if his land is farther away; and this will be in a case of necessity.
He said: When the Prophet (peace and blessings be upon him) received news that al-Harith bin Abi Dirar was gathering an army, he (peace and blessings be upon him) launched a night attack on him, while around him (peace and blessings be upon him) there were enemies who were nearer than al-Harith bin Abi Dirar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17881
Abdullah bin Abi Bakr (may Allah have mercy on him) said that the Prophet (peace and blessings be upon him) received news that Banu al-Mustaliq were preparing an army, led by al-Harith bin Abi Dirar, the father of the Mother of the Believers Sayyidah Juwayriyah (may Allah be pleased with her), so he (peace and blessings be upon him) set out. When he (peace and blessings be upon him) reached the well called al-Muraysi', one of the wells of Banu al-Mustaliq, they prepared to fight him (peace and blessings be upon him), and the people began to close in on one another. Then there was fighting. The Messenger of Allah (peace and blessings be upon him) defeated Banu al-Mustaliq. Some of them were killed, and the Messenger of Allah (peace and blessings be upon him) took their children, wealth and women as spoils, and he had attacked them from the side of the coast of Qudayd. Ibn Ishaq (may Allah have mercy on him) said: This took place in 6 AH.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17882
Ibn Awn says: I wrote to Nafi' asking him what the ruling is on the call (inviting to Islam) before fighting. He wrote back: That was only at the beginning of Islam. The Messenger of Allah (peace and blessings be upon him) launched a raid on Banu al-Mustaliq while they were unaware and their livestock were being watered at the water; the men fit for fighting were killed and the children were taken captive. On that day he obtained (I think he said) Sayyidah Juwayriyah bint al-Harith (may Allah be pleased with her). Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrated this hadith to me, and he was in that army.
Imam al-Shafi'i (may Allah have mercy on him) says: When news came that Khalid bin Sufyan bin Nubayh was gathering an army, Ibn Unays (may Allah be pleased with him) was sent towards him. He killed him, even though there were enemies near him as well.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17883
The son of Sayyiduna Abdullah bin Unays (may Allah be pleased with him) narrates from his father that he said: The Messenger of Allah (peace and blessings be upon him) sent me to Khalid bin Sufyan, who was in the direction of Uranah and Arafat, and he (peace and blessings be upon him) said: "Go and kill him." He says: I saw him, and at that moment the time of the Asr prayer came. I thought: I fear that the matter between me and him will cause the prayer to be delayed. So I set off towards him praying as I walked, making gestures (for bowing and prostration). When I came near him, he said to me: Who are you? I said: An Arab; news has reached me that you are gathering an army against this man, so I too have come to you for that purpose. He said: I am doing exactly that. So I walked with him for a while, and when I got my chance against him, I attacked him with my sword until he died.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17884