The Messenger of Allah ﷺ said: “The beginning of this matter is Prophecy and mercy, then it will be Caliphate and mercy, then it will be Kingship and mercy, then there will be Imarah and mercy, then they will fight over authority like fighting donkeys biting one another. So hold fast to jihad, and the best of your jihad is Ribat (i.e. guarding the frontier in the face of enemies), and the best and most virtuous of your Ribat is ‘Asqalan (Ashkelon).”
Related by al-Tabarani on the authority of Ibn ‘Abbas (r) in al-Mu‘jam al-Kabir #11138. Al-Haythami said ‘it’s men (i.e. narrators) are trustworthy’ (Majma‘ al-Zawa’id, 5/193)
قال الطبراني – رحمه الله – في “المعجم الكبير” (11138) : حَدَّثَنَا أَحْمَدُ بْنُ النَّضْرِ الْعَسْكَرِيُّ ، ثنا سَعِيدُ بْنُ حَفْصٍ النُّفَيْلِيُّ ، ثنا مُوسَى بْنُ أَعْيَنَ ، عَنِ ابْنِ شِهَابٍ ، عَنْ فِطْرِ بْنِ خَلِيفَةَ ، عَنْ مُجَاهِدٍ ، عَنِ ابْنِ عَبَّاسٍ ، قَالَ : قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ : ( أَوَّلُ هَذَا الْأَمْرِ نُبُوَّةٌ وَرَحْمَةٌ ، ثُمَّ يَكُونُ خِلَافَةً وَرَحْمَةً ، ثُمَّ يَكُونُ مُلْكًا وَرَحْمَةً ، ثُمَّ يَكُونُ إِمَارَةً وَرَحْمَةً ، ثُمَّ يَتَكادَمُونَ عَلَيْهِ تَكادُمَ الْحُمُرِ فَعَلَيْكُمْ بِالْجِهَادِ ، وَإِنَّ أَفْضَلَ جهادِكُمُ الرِّبَاطُ ، وَإِنَّ أَفْضَلَ رباطِكُمْ عَسْقَلَانُ )
Although Ashkelon is now the name of a coastal city in the Southern District of ‘Israel’, in the past, after the Islamic conquest of Palestine, ‘Asqalan was synonymous with Gaza (غزة). In fact, as mentioned by Shaykh Salih al-Munajjid (see here), the area was known as “غزة عسقلان” (“Gaza Ashkelon”).
At the time of writing, Gaza is facing a land invasion. Almost 3,000 children have been murdered by ‘Israel’. حسبنا الله ونعم الوكيل – “Allah is sufficient for us, and He is the best Protector” [Qur’an 3:173]
The Caliphate 
Can you give a reference too
For the history of the area, the reference is an Arabic article on “Gaza and Ashkelon from the beginning of the Abbasid era until the Crusader invasion” by Dr. Zuhair ‘Abdullah Sa’eed Abu Rahma (pp. 2-5).
الحياة العلمية في غزة وعسقلان منذ بداية العصر العباسي حتى الغزو الصليبي (ص2-5) د/ زهير عبد الله سعيد أبو رحمة
If this is a reliable hadith, why don’t we hear it mentioned more often?
Assalamu ‘alaykum wa rahmatu’Llah. Perhaps because there are more authentic hadiths on the subject, such as the following related in Musnad Ahmad:
The Messenger of Allah ﷺ said: “Prophethood will last with you for as long as Allah wants it to last. Then there will be Caliphate according to the Method of Prophethood, and things will be as Allah wishes them to be. Then Allah will end it when He wishes. Then there will be hereditary rule, and things will be as Allah wishes them to be. Then Allah will end it when He wishes. Then there will be an oppressive rule, and things will be as Allah wishes them to be. Then Allah will end it when He wishes. Then there will be a Caliphate according to the method of Prophethood.” Then he ﷺ fell silent.