Very weak
graded by Al-Albani
Mishkat al-Masabih 1442
وَعَنْ جَعْفَرِ بْنِ مُحَمَّدٍ مُرْسَلًا أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَأَبَا بَكْرٍ وَعُمَرَ كَبَّرُوا فِي الْعِيدَيْنِ وَالِاسْتِسْقَاءِ سَبْعًا وَخَمْسًا وَصَلَّوْا قبل الْخطْبَة وجهروا بِالْقِرَاءَةِ. رَوَاهُ الشَّافِعِي
Ja'far b. Muhammad told in mursal form that the Prophet, Abu Bakr and ‘Umar said “God is most great” at the two festivals and when praying for rain seven times in the first rak'a and five times in the second. They prayed before the sermon, and recited the Qur’an in a loud voice. Shafi‘i transmitted it.
Cited in Mishkat al-Masabih (No. 1442).