Commentary
It was said in verse 17 وَاذْكُرْ عَبْدَنَا دَاوُودَ ذَا الْأَيْد (and remember Our servant, Dawud, the man of might). Nearly all commentators have explained it in the sense that he used to demonstrate great courage and strength during the course of his acts of worship (` ibadah), therefore, soon after it, comes the sentence: إِنَّهُ أَوَّابٌ (Surely, he was ever-turning to Allah - 38:17). Accordingly, in a Hadith of the Sahihayn (al-Bukhari and Muslim), the Holy Prophet ﷺ said: "In sight of Allah, the most liked Salah is the Salah of Dawud (علیہ السلام) and the most liked fasts are the fasts of Dawud (علیہ السلام) . He would sleep half a night, worship a third of it, and then sleep during the sixth part, and would fast one day and remain without fast on the other; and when he confronted the enemy, he would never desert the battlefield; and, without any doubt, he turned to Allah sincerely and profusely." (Tafsir Ibn Kathir)
This method of ` ibadah has been called most likeable for the initial reason that it is harder. A lifetime of fasting would make one used to it with the result that, after a passage of time, it does not remain that hard. But fasting by skipping a day in between is different. Here, the element of hardship continues. Then, there is another aspect of this method of fasting in that one who fasts in that manner becomes capable of combining his or her ` ibadah with the necessary fulfillment of the rights of one's self, family and circle of acquaintances.