Salam aleikum.
After my istinja , from my sexual organ, drops come out. Is it urine?Must i repeat my istinja? I redo only wudu, but I do not wash my organ and my garments. Are my prayers valid? Or is it only water from istinja, the drops wich comes out?
I have another question, I know the hanabila see wujub saying dua between sajdatain. Is it obligatory to say rabbi ghfirli or i can say any dua of forgiveness?. If one forgives rabbi ghfrli and say another dua of forgiveness, is it correct?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

You are required to perform Istinjaa' (cleaning oneself after urination or defecation) in a normal way, and if you find drops after that and you are certain that those drops are urine, then you have to repeat Istinjaa' and wash the clothes that were touched by those drops. If you are not certain that it is urine, then do not pay attention to it, as your clothes are considered pure, your ablution is correct and your prayer is valid.

As for your second question we apologize for not answering it, as per the policy of the site, we only answer one question at a time.

Allah knows best.