What is the reference of Hadith which approximately means that a blessed leader and blessed army will conquer Istanbul
Perhaps you mean this hadith:
لتفتحن القسطنطينية، ولنعم الأمير أميرها، ولنعم الجيش ذلك الجيش
Verily you shall conquer Constantinople. What a wonderful leader will that leader be, and what a wonderful army will that army be!
Recorded in, for example: