An army from the Ansar was stationed in the land of Persia with their commander. Umar used to rotate the armies every year, but he became preoccupied and forgot about them. When their term had passed, the people of that frontier post returned home. Umar was harsh with them and threatened them, even though they were companions of the Messenger of Allah ﷺ.They said, "Umar, you neglected us and abandoned what the Messenger of Allah ﷺ commanded regarding the rotation of some military expeditions by others."
أن جيشا من الأنصار كانوا بأرض فارس مع أميرهم، وكان عمر يعقب الجيوش في كل عام فشغل عنهم عمر، فلما مر الأجل قفل أهل ذلك الثغر، فاشتد عليهم وتواعدهم، وهم أصحاب رسول الله ﷺ فقالوا: يا عمر إنك غفلت عنا، وتركت فينا الذي أمر به رسول الله ﷺ من إعقاب بعض الغزية بعضا.
Abu Dawud (2960) and Ibn Jarud (1095) both narrated it through Ibrahim bin Saad, from Ibn Shihab, from Abdullah bin Kaab, who mentioned it. The wording is that of Abu Dawud. Its isnad is sahih.Khattabi said, "I'qab means that the imam sends after those stationed at the frontier an army to take their place, so that those original troops can return home. For if their absence and separation from their families is prolonged, they suffer harm from it, and their families suffer harm as well."