ESPN
Abdullah, a practicing Muslim, dropped to his knees after scoring for what appeared to be a very subdued touchdown celebration by NFL standards. An official on the sideline can be seen almost immediately throwing a flag for what was called "unsportsmanlike conduct" for going to his knees.
The NFL bans "prolonged or excessive celebrations." These acts are to be penalized as "unsportsmanlike."
However, former NFL vice president of officiating Mike Pereira, now a commentator for Fox Sports, said that this should not be a penalty.
Abdullah's agent was not happy with the call and made it clear that his client should not be fined for his religious moment.
It would seem that the NFL probably wouldn't want to engage in religious restriction and it would be surprising if Abdullah is fined, especially when Christian players pray on the field quite often.
More likely, we will see clarification from the NFL that this should not have been penalized and players will be free to express their beliefs moving forward.