In electronic messages found on his phone:

“jihad is fard-ayn,” which translates to “jihad is obligatory.”Stephanie Clifford, “4th Brooklyn man indicted in plot to assist ISIS pleads not guilty,” New York Times, April 8, 2015, http://www.nytimes.com/2015/04/09/nyregion/4th-brooklyn-man-indicted-in-plot-to-assist-isis-pleads-not-guilty.html.