"They said, 'We did not fail your promise by our own will,'" meaning: by our own power, up to His saying: "and he forgot."
Yahya said: Their appointment with Moses was for forty nights. They counted twenty days and twenty nights, then said, 'These are forty, so Moses has failed us in the promise.' They had borrowed from
﴿قَالُوا مَا أَخْلَفْنَا مَوْعِدَكَ بِمَلْكِنَا﴾ أَيْ: بِطَاقَتِنَا إِلَى قَوْلِهِ: ﴿فَنَسِيَ﴾.
قَالَ يَحْيَى: كَانَ وَعْدُهُمْ مُوسَى أَرْبَعِينَ لَيْلَةً، فَعَدُّوا عِشْرِينَ يَوْمًا وَعِشْرِينَ لَيْلَةً، فَقَالُوا: هَذِهِ أَرْبَعُونَ، فَقَدْ أَخْلَفَنَا مُوسَى الْوَعْدَ، وَكَانُوا اسْتَعَارُوا مِنْ
