That passport belonged to Ganesh Gaitonde. Sartaj got that passport from Gaitonde's pocket. The passport was a big proof which shows that he is Ganesh Gaitonde, the very big goon.
Till that date, Sartaj was solving very small criminal cases of thieves. In one of his statement, he was saying to Katekar -
"10 sal me ek sonu pakit maar ko chodke kisko pakda hai maine! (who I've caught in ten years except for Sonu, a pickpocketer)".
Now, he has got the big clue of a big case through Gaitonde's phone call even though he has committed suicide. Sartaj is an honest police officer, and he was waiting to become in-charge of the big criminal case so that he could solve it and earn more prestige in his profession. It was Sartaj's dream. In that series, Gaitonde has shown as a very big goon. Finding Gaitonde (even though he was dead) and solving this criminal case is a big opportunity for Sartaj so as to make big career progress. Now, his dream will be going to become true.
Sartaj was saying to Katekar
"Do you believe in God?"
It means, God has listened to us and we have got big criminal case now to solve. Again, Sartaj was handing over the passport to Katekar because the passport was a big proof that the person who has committed suicide is Ganesh Gaitonde.