إِيلَافِهِمْ رِحْلَةَ الشِّتَاءِ وَالصَّيْفِ
"Acquaint them with the winter and summer trip"
English - translation with scientific references:
1-Their accustomed security [in] the caravan of winter and summer - Tafheem-ul-Quran by Syed Abu-al-A'la Maududi.
2-(Li-ilaf-i) Accustomed to the journey of winter and summer.​

Tafheem-ul-Quran by Syed Abu-al-A'la Maududi
​"The word ilaf, as used in the original is from Alf which means to be habituated and accustomed to being reunited after breaking up, and to adopt something as a habit. About the lam that is prefixed to ilaf, some Arabists have expressed the opinion that it is to express surprise and wonder.
​Thus, Li-ilaf-i Quraish means: "How surprising is the conduct of Quraish! It is only by virtue of Allah's bounty that they are reunited after their dispersion and have become accustomed to the trade journeys which have brought them their prosperity".

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"And yet from Allah's worship and service they are turning away. 'This is the opinion of Akhfash, Kisa'i and Farra', and holding this opinion something after this lam, the same thing itself is regarded as sufficient to show that the attitude and conduct a person has adopted in spite of it, is surprising and amazing".

On the contrary, KhaIil bin Ahmad, Sibawaih and Zamakhshari say that this is the lam of to `lil and it relates to the following sentence: Fa! ya `budu Rabba hadh a!-Bait, which means: "Allah's blessings on the Quraish are countless. But if for no other blessing, they should worship Allah at least for this blessing that by His bounty they became accustomed to the trade journeys, for this by itself is indeed a great favour of Allah to them".