The people of Kufa complained about Saad to Umar, saying that he did not pray properly. Umar questioned him about it.Saad replied, "I lead them in the prayer of the Messenger of Allah ﷺ. I prolong the first two units and shorten the last two."Umar said, "That is what I expected of you, Abu Ishaq."
حدثنا عبد الرزاق أنبأنا سفيان عن عبد الملك بن عمير
شكا أهل الكوفة سعدا إلى عمر فقالوا لا يحسن يصلي قال فسأله عمر فقال إني أصلي بهم صلاة رسول الله ﷺ أركد في الأوليين وأحذف في الأخريين قال ذلك الظن بك يا أبا إسحاق
Its isnad is sahih. Sufyan is Thawri, and the hadith is a repetition of 1510. "I stand still in the first two" means he remains motionless and prolongs the standing in the first two units of the four-unit prayer. "And I shorten in the last two" means he lightens them.