If you forget to sit for the second rakat after completing one rakat of Maghrib prayer with the imam 9858338535

In the case in question the rule was to sit after one rakat. But even if he recites two rakat directly without sitting, the prayer is completed. If someone forgets to do so, Sahu-Sijdah will not be wajib on him. So your prayer has become Sahih. Hazrat Ibrahim Nakhaee Rah. Narrated from, he said- أَدْرَكَ مَسْرُوقٌ وَجُنْدُبٌ رَكْعَةً مِنَ الْمَغْرِبِ, فَلَمَّا سَلَّمَ الإِمَام قَامَ مَسْرُوقٌ فَأَضَافَ لَيْهَا رَكْعَةً ثُمَّ جَلَسَ, وَقَامَ جُنْد ُبٌ فِيهَما جَمِيعًا ثُمَّ جَلَسَ فِي اخِرِهَا, فَذَكَرَ ذَلِكَ لِعَبْدِ اللهِ, فَقَالَ : كِلاَهُمَا قَدْ أَحْسَنَ وَأَفْعَلُ كَمَا فَعَلَ مَسْرُوقٌ اَحَبُّ إلَيَّ Hazrat Jundub and Masrook. One rakat with the imam in Maghrib prayer one day. After the salam of the imam, the masruk rah. After standing and reading one rakat, sit for Tashahhud. But Jundub Rah. Did not sit then; Rather, he sits for tashahhud at the end of the prayer. After that, they mentioned the matter to Hazrat Abdullah Ibn Mas'ud, he said, "The prayers of both of you have been completed." But in this case I prefer to do as Masrook did. (Musannaf Ibn Abi Shaiba, Narration No. 8569) 9232375716