بِخَلَـقِهِمْ
(their portion), means, they mocked their religion, according to Al-Hasan Al-Basri. Allah's statement,
وَخُضْتُمْ كَالَّذِي خَاضُواْ
(and you indulged in play and pastime as they indulged in play and pastime), indulged in lies and falsehood,
أُوْلَـئِكَ حَبِطَتْ أَعْمَـلُهُمْ
(Such are they whose deeds are in vain), their deeds are annulled; they will not acquire any rewards for them because they are invalid,
فِي الدنْيَا وَالاٌّخِرَةِ وَأُوْلَـئِكَ هُمُ الْخَـسِرُونَ
(in this world and in the Hereafter. Such are they who are the losers.) because they will not acquire any rewards for their actions. Ibn `Abbas commented, "How similar is this night to the last night,
كَالَّذِينَ مِن قَبْلِكُمْ
(Like those before you...) These are the Children of Israel, with whom we were compared. The Prophet said,
«وَالَّذِي نَفْسِي بِيَدِهِ لَتَتَّبِعُنَّهُمْ حَتَّى لَوْ دَخَلَ الرَّجُلُ مِنْهُمْ جُحْرَ ضَبَ لَدَخَلْتُمُوه»
(By He in Whose Hand is my life! You will imitate them, and even if a man of them entered the den of a lizard, you will enter it likewise!)" Abu Hurayrah narrated that the Messenger of Allah ﷺ said,
«وَالَّذِي نَفْسِي بِيَدِهِ لَتَتَّبِعُنَّ سَنَنَ الَّذِينَ مِنْ قَبْلِكُمْ شِبْرًا بِشِبْرٍ وَذِرَاعًا بِذِرَاعٍ، وَبَاعًا بِبَاعٍ حَتَّى لَوْ دَخَلُوا جُحْرَ ضَبَ لَدَخَلْتُمُوه»
قالوا: ومن هم يا رسول الله، أهل الكتاب؟ قال:
«فَمَنْ؟»
(By He in Whose Hand is my soul! You will follow the traditions of those who were before you a hand span for a hand-span and forearm's length for forearm's length, and an arm's length for an arm's length. And even if they enter the den of a lizard, you will also enter it.) They asked, "Who, O Allah's Messenger, the People of the Book" He said, (Who else)" This Hadith is similar to another Hadith collected in the Sahih.
Tafsir Ibn Kathir (abridged, English).