Mr.daood, you can take taxi from the hotel to Niujie mosque directly, it takes around CNY30.00. Or you may take subway line 1 at Guomao Station, and transfer to subway line 2 at Jianguomen Station. Getting off at Changchunjie Station, you could take taxi to Niujie, and the taxi fee is around CNY15.00. Hope it helps.

Walk to Guomao Metro Station. Take Subway Line 1 from Guomao to Xidan and then transfer to Bus No.626 at Xidan. Get off at Niujielibaisi (Niujie Mosque). Good Luck!