"And when they regretted," meaning: they became remorseful, "and saw that they had gone astray," the verse. They said that when Moses did to the calf what he did, and they sought repentance, but Allah refused to accept it from them unless they killed themselves; and the explanation of this has already passed in Surah al-Baqarah.
Muhammad said: It is said to one who regrets what he has done: 'He has fallen into his own hand' (suqita fi yadihi), or 'It has been cast into his hand' (usqita fi yadihi).
﴿وَلما سقط فِي أَيْديهم﴾ أَيْ: نَدِمُوا ﴿وَرَأَوْا أَنَّهُمْ قَدْ ضلوا﴾ الآيَةُ. قَالُوا ذَلِكَ لِمَا صَنَعَ مُوسَى بِالْعِجْلِ مَا صَنَعَ، وَطَلَبُوا التَّوْبَةَ، وَأَبَى اللَّهُ أَنْ يَقْبَلَ مِنْهُمْ، إِلا أَنْ يَقْتُلُوا أَنْفُسَهُمْ؛ وَقَدْ مَضَى تَفْسِيرُ هَذَا فِي سُورَةِ الْبَقْرَةِ.
قَالَ محمدٌ: يُقَالُ لِلْنَادِمِ عَلَى مَا فَعَلَ: قَدْ سُقِطَ فِي يَدِهِ، وَأُسْقِطَ فِي يَده.