Surah al-A`raf (The Elevated Places) 7 : 52

A study of its contents clearly shows that the period of its revelation is about the same as that of Surah 6: al-An’am (The Grazing Livestock), i.e. the last year of the Prophet's life at Makkah, but it cannot be asserted with certainty which of these two were sent down earlier. The manner of its admonition clearly indicates that it belongs to the same period. [Ref: Mawdudi]

It is considered the longest surah revealed during the Makkan period. Some consider this surah to have been revealed after Surah 38: Sad. [Ref: Tafsir al-Maudheei, Dr. Mustafah Muslim, vol. 3, p. 2]

• The Prophet Muhammad [saw] said,
  وأنَا تاركٌ فيكمْ ثَقَلَيْنِ : أولُهما كتابُ اللهِ فيه الهدى والنُّورُ، منْ استمسكَ بهِ وأَخَذَ بهِ كانَ على الهُدى، ومنْ أخطأَهُ ضَلَّ، فخُذُوا بكتابِ اللهِ تعالى، واسْتَمْسِكُوا بِهِ؛ وأهلُ بيتِي، أذكِّرُكم اللهَ في أهلِ بيتِي، أذَكِّرُكم اللهَ في أهلِ بيتِي
  “...And I have left you two heavy [important issues]: The first of them is the Book of Allah. In it is the Guidance and the Light. Whoever holds firmly to it and takes from it he [then] is upon the Guidance. Whoever errs with respect to it goes astray. So take from the Book of Allah Almighty and hold firmly to it. [the second thing] is Ahl – Bayti [My Family – included wives, children, son-in law etc] I remind you by Allah with regard to My Family, I remind you by Allah with regard to My Family.  [Muslim, Ryadh as-Saliheen no. 346, – صحيح [Sahih]

• The Prophet Muhammad [saw] said, إنِّي قد ترَكْتُ فيكم ما إنِ اعتصمتُمْ بِهِ فلن تضلُّوا أبدًا كتابَ اللَّهِ وسنَّةَ نبيِّهِ “I leave behind for you, that by which if you stick firmly to you will never go astray, the Book of Allah and the Sunnah of His Prophet.” [Targhib wa Tarhib 1/61  صحيح [Sahih]