Hadith regarding the Muslim Ummah

Q: Please enlighten me with the references of this hadith: Conjoined Hadiths from Musnad Ahmad: Jaabir (radhi-yallaahu 'anhu) narrated that Allaah's Messenger (sal-Allaahu`alayhe wa sallam) said: "Al Muslimoon Ummatun Wahida, Wa Biladahum Wahida, Wa Harbahum Wahida, Wa Salmahum Wahida". Is this a Hadith at all?

Hadith regarding perfect Imaan

Q:

  1. I read that no one will enter jannah unless they have perfect imaan. So if a person has Imaan but it is not perfect will he attain jannah? Quote hadith or quraan ayaats if possible.
  2. What is perfect imaan? If I commit a sin does it mean I do not have perfect imaan ?
  3. Can you send me the hadith which says how one can attain perfect imaan through salaam?