I'd like to add to the nice post of brother Zia that this hadith is also quoted in Jami' al-Ulum wal-Hikam جامع العلوم والحكم of ibn Rajab al-Hanbali. This book is including the ahadith of imam an-Nawawi's 40 hadith and completing it to 50 hadiths beside comments on each hadith.
Ibn Rajab said that this hadith has two issue's:
First that the hadith is based on a narration of (the tabi'iy) Makhool مكحول from (the sahabi) abi Tha'labah al-Khoshni, but there's no evidence that Makhool ever heard or met abu Tha'labah according scholars like abu Na'im and others!
Secondly that scholars are not in consensus whether this is a hadith marfo' or not.
Ibn Rajab qualified it as hassen because of other narrations which support it like a hadith of abu a-Darda'a compiled by al-Bazzar (in his musnad) and al-Hakim (in his mustadrak)... (my own translation take it carefully!)
عن النبي صلى الله عليه وسلم ، قال : ما أحل الله في كتابه فهو حلال ، وما حرم فهو حرام ، وما سكت عنه فهو عفو ، فاقبلوا من الله عافيته ، فإن الله لم يكن لينسى شيئا ثم تلا هذه الآية : وما كان ربك نسيا [ مريم : 64 ]
...the Messenger of Allah said:
What Allah made halal in his book is halal, and what HE made haram is haram, And what ever He left unmentioned is a mercy (to you), so accept from Allah HIS mercy, as Allah is not forgetting any thing and he recited the verse "And never is your Lord forgetful -" (19:64)
Which has also been compiled by at-Tabarni and a-Daraqotni on the authorithy of abu a-Darda' with a similar statement as the one of abu Tha'labah ending with: " رحمة من الله فاقبلوها mercy from Allah so accept it" with a weak sanad.
Also at-Thrimdihi and ibn Majah have compilled a hadith with a similar meaning on the authority of Salman al-Farisi and abu Dawod on the authority of ibn 'Abbas. At-Thirmidhi qualified his narration as gharib.
So it is a hadith which is only hassan by support of other narrations, but the content covers the four rulings Allah gave us: orders of Allah, what Allah made haram, the hodod (limits) Allah defined, and what is unmentioned.