The people of Kufa complained about Saad to Umar, saying that he did not pray properly. Umar mentioned this to him.Saad replied, "As for the prayer of the Messenger of Allah ﷺ, I would lead them in it. I would prolong the first two units and shorten the last two."Umar said, "That is what I thought of you, Abu Ishaq."
حدثنا جرير بن عبد الحميد عن عبد الملك بن عمير
شكا أهل الكوفة سعدا إلى عمر فقالوا لا يحسن يصلي فذكر ذلك عمر له فقال أما صلاة رسول الله ﷺ فقد كنت أصلي بهم أركد في الأوليين وأحذف في الأخريين فقال ذاك الظن بك يا أبا إسحاق
Its isnad is sahih. Jareer bin Abdul-Hameed bin Qurt Dabbi is trustworthy, an authority, and a hadith master. Ahmad narrated from him repeatedly, including in this place and in 177. The hadith is a repetition of 1584.