Muhammad bin Ishaq bin Yasar mentions the story of Khalid bin al-Walid (may Allah be pleased with him) after the Battle of Yamamah was over: Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) wrote to Khalid bin al-Walid (may Allah be pleased with him) while he was still in Yamamah: From Abu Bakr al-Siddiq (may Allah be pleased with him), the successor (khalifah) of the Messenger of Allah (peace and blessings be upon him), to Khalid bin al-Walid (may Allah be pleased with him), the Muhajirun and the Ansar (may Allah be pleased with them) and those who follow them. Peace be upon you. I praise Allah, besides whom there is no god. To proceed: All praise is for the One who fulfilled His promise, helped His servant, gave might to His friend, humiliated His enemy and alone overcame the confederates. Indeed, Allah is the One besides whom there is no god: "Allah has promised those of you who believe and do righteous deeds that He will surely make them successors in the land, just as He made those before them successors, and that He will surely establish for them their religion which He has approved for them" [Surah al-Nur: 55].
He recited the entire verse and wrote it. It is a promise from Him that is never broken and a word in which there is no doubt. And He has made jihad obligatory upon the believers; He said: "Fighting has been prescribed for you, though it is hateful to you" [Surah al-Baqarah: 216]. When he had finished the verses, he said: Seek the fulfilment of Allah's promise to you, and obey Him in what He has made obligatory, even if it involves severe hardship and the distance is far. You have been tried in your lives and your wealth; that is little compared with the great reward of Allah. May Allah have mercy on you! Go forth to fight in the path of Allah, "light and heavy, and strive with your wealth and your lives" [Surah al-Tawbah: 41]. Beware! I have ordered Khalid bin al-Walid (may Allah be pleased with him) to march to Iraq; remain there until my order comes. You are to go with him and not lag behind, for this is the path of Allah in which the reward is great for one whose intention is good and whose desire for good is great. When you reach Iraq, remain there until my order comes. May Allah suffice us and you in the important affairs of this world and the Hereafter. Peace, the mercy of Allah and His blessings be upon you.
The Shaykh (may Allah have mercy on him) says: Then it is explained in the histories that he wrote (to Khalid) to march to Syria and wrote to the commanders of the other armies to assist him; then the victory the Muslims were granted on the day of Ajnadayn in the time of Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him), the departure of Heraclius towards Rome, the conquests that took place there and in Iraq and Persia, the destruction of Kisra, and the carrying of Kisra's treasures to Madinah in the time of Sayyiduna Umar (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18610