Tafsir of Al-Quraish 106:2

Their Ilaf caravans, in winter and in summer. Ibn Jarir said, "The correct opinion is that the letter Lam is a prefix that shows amazement. It is as though He (Allah) is saying, `You should be amazed at the uniting (or taming) of the Quraysh and My favor upon them in that.''' He went on to say, "This is due to the consensus of the Muslims that they are two separate and independent Surahs.'' Then Allah directs them to be grateful for this magnificent favor in His saying,

فليعبدوا رب هذا البيت