Are there any Hadith indicating that praying Isha in congregation at the mosque is considered equivalent to performing Qiyam (standing in prayer) for half the night if the worshippers leave after the Imam, and is it authentic that the Prophet, peace and blessings be upon him, used to say "Allahu Akbar" after concluding the prayer with "Salam" and before seeking forgiveness?

"Whoever prays Isha in congregation, it is as if he has stood (in prayer) for half the night. And whoever prays Fajr in congregation, it is as if he has prayed the entire night." And the notion that this reward is only attained by someone who leaves after the Imam departs has no evidence to support it. The Prophet, peace and blessings be upon him, would seek forgiveness three times after concluding the prayer, saying: "Astaghfirullah, Astaghfirullah" (I seek forgiveness from Allah, I seek forgiveness from Allah), and would follow it by saying: "Allahumma Antas-Salam wa minkas-salam, tabarakta ya Dhal-Jalali wal-Ikram" (O Allah, You are As-Salam, and from You comes peace. Blessed are You, O Possessor of Majesty and Honor).