Talhah bin Abdullah bin Abd al-Rahman bin Abi Bakr al-Siddiq (may Allah be pleased with them) said: When Sayyiduna Abu Bakr (may Allah be pleased with him) dispatched armies against the apostates, he would instruct the commanders: When you attack a territory, raid it from every side; kill, burn, and be thorough in killing and wounding. Let no weakness appear in you because of the death of the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18124