Rukn (ركن) plural ʾArkān (أَرْكَانٍ)
The Corners of Islam
قرب الإسناد ابْنُ عِيسَى عَنِ الْبَزَنْطِيِّ قَالَ سَمِعْتُ الرِّضَا ع يَقُولُ الْإِيمَانُ أَرْبَعَةُ أَرْكَانٍ التَّوَكُّلُ عَلَى اللَّهِ عَزَّ وَ جَلَّ وَ الرِّضَا بِقَضَائِهِ وَ التَّسْلِيمُ لِأَمْرِ اللَّهِ وَ التَّفْوِيضُ إِلَى اللَّهِ قَالَ عَبْدٌ صَالِحٌ وَ أُفَوِّضُ أَمْرِي إِلَى اللَّهِ … فَوَقاهُ اللَّهُ سَيِّئاتِ ما مَكَرُوا
(The book) ‘Qurb Al Asnaad’ – Ibn Isa, from Al Bazanty who said,
‘I heard Al-Reza-asws saying: ‘The Eman is of four ‘أَرْكَانٍ’ corners – the reliance upon Allah-azwj Mighty and Majestic, and the satisfaction with His-azwj Decree, and the submission to the Command of Allah-azwj, and the delegating (affairs) to Allah-azwj. A righteous servant said: ‘and I entrust my matters to Allah, [40:44] So Allah Saved him from the evil of what they planned, [40:45].[1]
[1] Bihar Al-Anwaar V 67 – The book of Eman and Kufr – Ch 63 H 13