in your question.

What were the people of Kufa complaining about when they spoke to Umar (ra) about Sa'd bin Abi Waqqas (ra)?
 Narrated Jabir bin Samura:                     The People of Kufa complained against Sa`d to `Umar and the latter dismissed him and appointed `Ammar as their chief . They lodged many complaints against Sa`d and even they alleged that he did not pray properly. `Umar sent for him and said, "O Aba 'Is-haq! These people claim that you do not pray properly." Abu 'Is-haq said, "By Allah, I used to pray with them a prayer similar to that of Allah's Apostle and I never reduced anything of it. I used to prolong the first two rak`at of `Isha prayer and shorten the last two rak`at." `Umar said, "O Aba 'Is-haq, this was what I thought about you." And then he sent one or more persons with him to Kufa so as to ask the people about him. So they went there and did not leave any mosque without asking about him. All the people praised him till they came to the mosque of the tribe of Bani `Abs; one of the men called Usama bin Qatada with a surname of Aba Sa`da stood up and said, "As you have put us under an oath; I am bound to tell you that Sa`d never went himself with the army and never distributed (the war booty) equally and never did justice in legal verdicts." (On hearing it) Sa`d said, "I pray to Allah for three things: O Allah! If this slave of yours is a liar and got up for showing off, give him a long life, increase his poverty and put him to trials." (And so it happened). Later on when that person was asked how he was, he used to reply that he was an old man in trial as the result of Sa`d's curse. `Abdul Malik, the sub narrator, said that he had seen him afterwards and his eyebrows were overhanging his eyes owing to old age and he used to tease and assault the small girls in the way.